So...
I bought this comp about a year and a half ago:
 
Mobo: 1st mainboar AZ11EA
RAM: PC133 256mb
Processor: AMD Athlon 1.4ghz
Vid Card: Hercules Kyro II 64mb
OS: Win 2k prof.
A 30gig HD, can't remember brand.
Creative DVD, Plextor CD-R
 
It had problems.  Many of them.  The main one was the sudden and harsh freezing that would occur as seemingly random intervals.  I literally have replaced everything (mobo, processor, HD, vid card, cd-r burner...) in this computer (thank goodness for the warrenty) EXCEPT the power supply, and the RAM.  I now have:
 
Same mobo, RAM, cd drives
Vid Card: Nvidia GeForce 3 64mb
Processor: AMD Athlon 1.3ghz
HD: Quantum Fireball Plus AS 30gig
OS: Win XP Prof.
 
Downgrading my processor meant changing the FSB to 100mgz, and the PC133 RAM then adjusted.  With the installation of Win XP, the freezing has lessened from about once every 2 hours, to once every 6 days.  I didn't install XP until about a month ago.  Anyway, after this, the comp was slow.  This seemed odd, but it was taxing the processor 100% for simple things like copying files off CD, while running a very minimum of background programs.  So, with RAM so cheap these days, I bought a PC100 512mb stick, and here's my ultimate question:
 
Can I stick the PC100 stick in with the PC133 stick, or is that bad?  
 
And, further, if you know of any reasons as to why my computer might be freezing, (I can only assume it's the RAM), any help would be greatly appreciated.
 
<i>I'm gonna go eat a granola bar now.</i>

You can put your PC100 stick in there, but you'll have to run the RAM at 100MHz.
 
Your computer might be freezing because of bad drivers.  Your computer also might be freezing because of a bad power supply.  It could even be freezing because of bad RAM.
 
 
PC Repair-Vol 1:Getting To Know Your PC.
PC Repair-Vol 2:Troubleshooting Your PC.
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Most if not all the time...random lock ups are either cause by a failing power supply (voltage rails dropping below specs) or a bad stick of ram....so you basically replaced everything for no reason   ....most of the time if a motherboard,processor or video card is bad they just wont work period....hardware conflicts can also cause lockups especially with earlier via chipsets and the good ol soundblaster live......why would you downgrade from pc133 to pc100?...and better yet why get a 200fsb processor instead of a 266?...are you still having lockups after downgrading almost all your equipment?...what kind of power supply do you have?...what cas latencies are you running on this generic ram?
 
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I bought the pc100 just because my FSB was at 100, and, having tried many other things, i figured I'd get the RAM meant for the bus.  The main reason I downgraded the processor was because of a mistake made by the company I bought it from (which has since gone out of business, voiding my 3 year warrenty).  They told me over the phone about a week after I recieved the computer that they should have noticed this when they were building the comp that the mobo I had, the AZ11EA, couldn't support the processor I had ordered, the 1.4ghz.  They did not tell me exactly why.  I was given the choice of downgrading the processor, and getting about 50 bucks back, or upgrading the mobo, and spending about that much.  So I downgraded.  Since the 1.4ghz required a 133mhz bus, and the 1.3ghz required a 100, that's where the change came in.
 
Heres the info on the label of the power supply, as I know nothing of what is needed.
 
Very top covered by casing..
AC Input | 115\230V 50-60hz 6/3A Fuse Rating: T6.3Al,250V
DC Output | +3.3V | +5V | +12V | -5V | -12V | +5Vsb
Max. A    | 16A   | 25A | 10A  | .5A | .5A  | 1.5A
+5V&+3.3V Combined 165W  Peak Output 300W
Then the warning...
Then I think the serial number
Then
INTEL V.2.03
 
So yeah, the only thing actually downgraded was the processor, the rest was just replaced.  
 
Questions from the first reply:
How would I go about changing the ram to 100mgz, does it do it by itself?
 
Questions for civilized:
I don't know what cas latencies are.  The PC133 256 was generic ram.  The new PC100 512 stick is "Major Brand - High Grade," as listed on the site I bought it from, www.1stchoicememory.com.  Oh yeah, and it's 168 pin.
 
Oh, and there's this, (This should have gone in the first post, I apologize...it just gets better and better!).  So, a few months ago my HD started making funky whiring noises, and then making a clicking noise, over and over.  While doing so, the comp slowed down and would not respond to any input, until either A) it stopped making the noises and went back to normal or B) it blue screened, did a mememory dump, said something about a Kernal_Stack_Inpage_Error, and I would have to restart.  (I apologize for not having the whole error messege, for this hasn't happened in a while, and here's why...)  So, I went to Maxtor's website, found the model number, found some test software, (powermax, i think).  Made a boot disk and tested it.  Sure enough, it found some error on the drive, but not before it froze a couple times doing the testing, and making me restart it.  I had recently reformatted the drive, and had trouble doing that too, so I figured there would be an error.  Anyways, it said it fixed there error.  I have not had any problems with it since.  This error fixing corresponded with the installation of win XP, and thus the less frequent freezing could very well come from this.  Anyway, since then the comp acted very slow, as if low on RAM.  And now we are up to date.
 
I have replaced the 256 stick with the 512 stick, and there is definently an improvement in the speed.  However, the slow problem still remains, (and this is what was happening before), that, say, I'm copying files from CD, or playing mp3s, I move the mouse around and the cursor jerks around like stop motion video.  This jerking happens at the same time the hard drive light goes on and the HD makes a sound.  Meanwhile, the task manager says 100% usage.
 
Well, thanks to all who have read this far, and I appreciate all who have responded!  Thanks a bunch.
 
<i>I'm gonna go eat a granola bar now.</i>

Haha, silly ge0.
 
1.) PC100 and PC133 are the same thing, except that PC133 is higher quality.
 
2.) Much PC133 that is found to be defective is retested at 100MHz.  If that hides the flaws, it's remarked as PC100 and resold
 
3.) Much normal PC133 has been remarked as PC100 in order to get people who don't believe the faster memory is compatable with their older system to buy it.  In fact the price has often been increased as well because it's "harder to find" memory after being remarked.  I refer to this as the "sucker fee".
 
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>

Damn straight!  If anyone thinks that's bad, check out the 'sucker fee' for PC66 markings!  Ha!
 
I'm perfectly happy to run PC133 at PC66 speeds.  I feel a little guilty at wasting perfectly good performance parts on such a cheezy system, but I'm still happy.  I suppose it could be worse.  I could have a GeForce card in the system.  **LOL**  Then I'd <i>really</i> feel guilty.
 
 
PC Repair-Vol 1:Getting To Know Your PC.
PC Repair-Vol 2:Troubleshooting Your PC.
PC Repair-Vol 3:Having Trouble Troubleshooting Your PC?
PC Repair-Vol 4:Having Trouble Shooting Your PC?

That power supply sounds pretty ... crappy.  Just quickly comparing Antec's ratings for their old low-end 300W supply to yours goes (Antec 300W / your 300W):
+5V = 30A / 25A
+3.3V = 20A / 16A
+12V = 15A / 10A
 
So it could just be your power supply.  It certainly doesn't sound like a good one.
 
But then you had to go and mention your hard drive problems.  (Which sounds very much like the culprit.)  If your hard drive had bad sectors, chances are it'll have more.  It sounds like either dust got in or the heads are scratching the platters.
 
The way that reformatting 'fixes' the errors is by mapping around them.  If new errors come in afterwords though (and chances are they will if the hard drive is already going bad) then you're screwed all over again.
 
So the first thing that I'd do if I were you is replace the hard drive.  If that doesn't fix it then I'd try replacing the power supply.
 
And welcome to OEM hell.
 
 
PC Repair-Vol 1:Getting To Know Your PC.
PC Repair-Vol 2:Troubleshooting Your PC.
PC Repair-Vol 3:Having Trouble Troubleshooting Your PC?
PC Repair-Vol 4:Having Trouble Shooting Your PC?

Awesome, you guys have been a great help.  I was planning to replace the HD anyway, with something a little bigger, (4 times bigger!), but money issues blah blah blah have halted that for a while.  When I ran the test prog. that I got from Maxtor, and it found the errors, to "repair" it, it certaintly didn't reformat it.  It just said, errors fixed, and it didn't even seem to take any amount of time.  Sure enough, I haven't had any problems with the drive expect for the slowness.  But still, I'm gonna get a new one.  (Get this, the drive I have is the replacement of the first drive I got because that first drive was failing me horribly, and upon inspection, I noticed some screws missing in the casing and I could actually see the disk.  Dust, dust, dust.)  
Since the input of the 512 stick, performance has increased greatly.  I thank you all for the information on the RAM.  I'll definently look into getting a better power supply as well.
 
Thanks again.
 
<i>I'm gonna go eat a granola bar now.</i>

Crash,
 
He is probably having power problems, you think.  With 10A on the 12V line that thing is probably choking.
 
Components on the 12V rail:
 
Motherboard: 0.3A
Processor Fan: 0.25A
System Fan: 0.25A
DVD-ROM: 0.8A
CD-RW: 0.8A
HDDs: 2.0A
 
Just on the other devices alone the total is ~4.4A.
 
Add a processor that draws at around 6A and bingo.
 
On an Model 6 XP processor, XP1600+ = 1400MHz, the max draw is 1.75V and 35.9A(Max). (See page 31 of the AMD Athlon™ XP Processor Model 6 Data Sheet for the specs.)
 
That is 62.825W... That divided by 12V = 5.235A added to the 12V rail...
 
He's at ~9.635A out of his 10A.
 
Your thoughts?
 
<b><font color=red>Fredi</font color=red> <font color=red>Fredi</font color=red> He's our man! If he can't do it no one can!</b>

I agree ....a quality power supply should always be a must in todays power hungry systems...gettign away with cheap generics is becoming a hard thing to do these days without the chance of having some sort of instabilities
 
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Could be a power issue yes, but I don't think anyone here needs my help on that issue!
 
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>

Hmm, seems that a new power supply is needed.  I know nothing of these, any suggestions?  I wouldn't know where to begin...how do I know if I buy the right kind etc.  And how much do these puppies go for?
 
Again, much thanks to everyone.
 
<i>I'm gonna go eat a granola bar now.</i>

Enermax are nice and common, the ones I prefer are sold under the brand names Aopen and Sparkle, I recommend AGAINST any Antec power supply except the TruePower series.  I'd say 350w, but you might want more to help "future proof" your system.
 
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>

I was typing this at work when I was distracted for about 2 hours.  Yours was the last post before I hit continue.
 
<b><font color=red>Fredi</font color=red> <font color=red>Fredi</font color=red> He's our man! If he can't do it no one can!</b>