I built my computer about 6 months ago and it has been rebooting particularly when I burn DVDs with my Sony DRU-710.  I have recently replaced my Antec 300W Power Supply with a Thermaltake TR2-430W with hopes of remedying the problem, but it still does it.  My system specs are:
 
Chaintech Summit SPT800 Socket 478 Motherboard (I think I skimped)
Intel Pentium 4 3.0 Ghz 1MB Cache 800 Mhz FSB w/ HT  
(should have gone Athlon 64, but so expensive when I built computer)
XFX 6600GT AGP Graphics Card OC'd to 540/1050  
(possibly upgrade to X800 GTO shortly?)
80 GB 7200 RPM Hitachi Parallel ATA HDD w/ 2 MB Cache
(should have gone with Serial ATA? but got deal on this one)
2 x 512 MB Corsair XMS Dual Channel DDR400 RAM (happy with RAM)
Sony DRU-710A DVD Burner
Antec SLK2600AMB Steel Mid Tower Case with intake and exhaust fans
Dell 19" LCD monitor w/ 16 ms response time
 
Half Life 2 also takes a long time to load, and it stutters at medium settings at 1024 x 768 resolution with no AA or AF.  I game, burn & watch DVDs, and use the computer for my college classes (casual use).  This was my first build, but I'm learning.  Any help or insight you could provide would be appreciated.

Reboots usually point to heat related issues. Make sure the CPU heatsink/cooler isn't broken or loose. Also if you have a CPU temerature monitor program see what your cpu temp is after playing HL2 for a few minutes.

Oh yeah, I forgot to list my Coolmax Socket 478 Cooper core CPU cooler in my system specs, and it is on and working properly.  I don't have any temp. monitoring devices besides my HDD Thermometer and it typically reads 86 to 89 degrees F.

This is a shot in the dark but, loose ram chip maybe? Slow load on hl means no ram.

System properties lists 1 GB of RAM.  I double checked anyway and both RAM chips were securely in their slots.  Is the system just not capable of good Half Life 2 performance?  I thought I'd seen some benchmarks that show I should get pretty good framerates.  I still have no idea about the restarts though.  Thanks for the input so far.  I am at a loss...

Don't forget power supply unit, my cpu had problems like this. Though it was heat, wasn't. Corrupt RAM? mmm was PSU first dust bunnies were having the bulge down there, 2nd was just too old not working right to provide enough power.  
 
And your rig... mmm honestly my cpu is not far off from yours atm, 2.0 intel and lower end mobo and card, and 512 RAM =( but I can see your rig not being as good for HL2 as you'd want. Although shouldnt be terrible Framerates...
 
graphics card id upgrade think help a lot. And honestly cpu, intels just overheat and athlons play games so much smoother, HL2 is more CPU effected game too.

That system is beyond sufficient enough to run HL2 .. I've seen my friend do it with a Celeron 2.9ghz and Integrated Graphics
 
Ok first of all ... reasons maybe Heat, with CPU(Early p4 Prescott cores were pretty hot) and a solution to this can be (even if you have good thermal application, it may drop temps by another 5-10C)
 
http://overclockers.com/tips1187/
 
I own an A64-3000NewCastle (2ghz @2.4ghz - stock HSF), before it idled at around 45C no load and about 55C+/- on 100% load with Prime95
 
With the case mod, it dropped temps to 28C+/- Idle, and about 35C+/- 100% load with prime95, however I only used 1 80mm fan and it still remained very cool without the extra noise. I highly recommend doing this.
 
 
Next it may be your overclocked video card which may lead to heat/stability issues... as above you can use a cheap solution to provide adequate ventilation into your case.
Check video drivers, possibly corrupt and can cause stablity issues, I have friend who also had problems with games and a GeForce6600
 
 
I don't think it could be Ram b/c i have the same type and Corsair XMS ram is high quality and stable.
 
Powersupply could be a possiblity with the high power consumption of the p4 Prescott combined with external power going to your 6600, check and see if you have enough amps on your 12V+ rail on the powersupply, recommended minimum nowadays is 18A
 
 
But fix the little thing(s) first, like heat being a possiblity.

The video card driver check sounds like a good place to start.  I'm not very experienced in that area.  How do I know what they are supposed to be?  Would I get updated drivers from Nvidia's website?  The foam board and duct work option sounds like I could screw something up fairly easily.  Do you think this would really be a viable option if the driver check/update doesn't work?  It looks like it made a big difference for AntiHax0r, but looks fairly complicated.  As for my power supply, it has 18A on the 12V rail, 30A on the 5V rail, and 20A on the 3.3V rail.  Could that be the problem?  I just got it last month as it was highly recommended.

CommodoreRed said "HL2 is more CPU effected game"
 
That would explain why I can play BF2 fairly well, as I've heard it places more strain on the video card versus the CPU.  However, I figured 3Ghz would be enough to play HL2 at medium settings without skips and jitters.

Download memtest86, burn it onto a CDR, boot up and let it run for as long as possible.
 
If you have bad or flaky RAM memtest86 will find it!
 
http://memtest86.com/memtest86-3.2.iso.zip
 
Good luck and happy new year!

Okay, I'll try memtest86 and see how it does.  Thanks.

 
Yes get drivers from the Nvidia site, video drivers for new cards are typically unified drivers that support a range of cards from that company.. try several drivers and maybe give a third party driver a try.. www.omegadrivers.net
 
Your power supply appears to be good enough for your system.

Ok, I ran memtest86 and walked away from my computer for a couple of hours.  When I came back the power light was still green, but nothing was on.  It wouldn't boot after that for about 15 min.  I unplugged it and took out the RAM and reinserted the chips in their slots and it finally booted again.  I was quite scared.  Is that normal after memtest86 (I hope not).

What kind of chipset cooling do you have? I had the same problems before and it was the chipset cooling that was the problem. The stock cooling was not enough. So I changed the passive cooling on the chipset for a fan cooling and the problem was gone.

That's very interesting.  It is a passively cooled heatsink type cooler.  Is there any way to test to see if this is the problem or is changing it to a fan the only way to tell?  Also, I've never seen any chipset cooling fans.  I'm going to check Newegg now.

Memtest86 shouldn't crash your system.
 
I would suggest watching it if you can.
 
Hit SHIFT if the monitor turns off. It's possible your power management kicked in.

For testing you could take a normal table fan. Open the computers case and put the table fan blowing directly at the motherboard, if the problem disappears or you are able to keep the computer alive for longer then it's most likely a heating problem. And then it's very likely it's the chipset that has become more sensitive to heat.

Add this line in the game launch options for Half Life 2.
 
Open Steam then right click the Steam logo in the systems try and select the games list, go to the HL2 game and right click on it and find the LAUNCH Options.
 
Copy paste this line in.. For all your Steam Games.
 
-console -novid -dxlevel 8.0
 
This should solve your problem as your re-boot issue with HL2 and Steam is not an overheat problem it is a DirectX Diag error causing Windows XP to default to the safety shutdown reboot instead of the old Win98 Blue Screen. You can disable the XP re-boot by right clicking on My Computer and select properties it is in there under the Advanced TAB and Start-up and Recovery settings. Just uncheck the Automatically Restart check box.
 
If you still have problems fixing the Steam Source bug and still reboot after you uncheck that restart box then you could have a heat problem.
 
I think this should fix your problem as your graphics card is only a DX8 card at a guess as DX9 cards do not cause the XP re-boot problem you say is happening with Steam. Steam is just the host HL2 and Source are the product you run within the host but the Host (Steam) checks your system for compatibility.
 
P.S I almost forgot go get the latest graphics drivers for your ATI or Nvidia graphics card. Make sure your motherboard drivers are installed by checking again under hardware in a right click of the My Computer Icon. Any Yellow question marks you Might see there need to be resolved by installing the correct drivers for each device.

 
No, 6600GT is a DX9 card. It is a DX9.0c card to be exactly, all Geforce 6000-cards are DX9.0c.
 
But I can agree with you that this is most likely not a heating problem, I was tired earlier so I missread the post and believed the computer was rebooting when he was doing other things besides gaming.