I have installed Windows Vista Ultimate (64bit) on a system I built myself and I have had it crash (To a point where I can't use the mouse) twice in about 2-3 weeks. Is this normal for Vista? If not what could the problem be?

It's not normal, but I think we'd need more information to tell you what else it could be.  What are you doing when it crashes?  Have you checked the disk drives?  Run a test on your RAM?  Is the CPU cooler installed properly?  Are all your drivers updated?  Using any oddball beta drivers?
 
Honestly, without more information anything in terms of cause is going to be a wild guess.

The last time it crashed I opened ASUS temperature software and it asked me if I wanted to end it (was not responding) and I clicked wait to see if it responds and it crashed.
I think the other time it crashed I was just listening to music and surfing the internet.
 
I checked the RAM in my old PC and it was fine, no errors. The CPU is not overheating although it is hitting 50C.
I installed the drivers off the disk that came with my motherboard. I downloaded the Nvidia ones. I've read online that the Nvidia drivers cause alot of crashes. Maybe its them?

No one knows? Another problem I get is when I'm multitasking Windows Media Player jumps (The music skips). I had this problem with Windows ME and Windows 98SE and it stopped when I upgraded to Windows XP. Now its back with vista. By the way I have a fairly good spec PC so I don't think its a hardware problem.

I get the jumping music too, most notable when I am resizing or moving windows.  But I only get this after a warm boot/restart - do you get it after a cold boot. I get the same thing on XP. It has something to do with filling all four memory slots on my PC.
 
An occasional crash or non-responsive program is not very rare over a few week period.

Windows XP was fine for me at all times. I got the problem when I booted up my PC. I think thats a cold boot?  
I take it a cold boot is when the computer has not been on for an hour or so?
 
So its normal for the system to lock up if its rare and not all the time?
 
Cheers for the help.

 
 
Yes and possibly no. It may depend on your particular hardware/software/drivers/settings etc. as to how often exactly you get a problem but an occasional glitch is certainly par for the course. This is not to say that your problem is just a routine glitch. There may be a fix for you in updated drivers or a service pack or in an updated piece of software. I'm just saying that an unresponsive program cropping up from time to time is definitely not unusual.  Sometimes you can find fixes and sometimes you can't. You may have just been lucky with your XP system. XP systems crash too, it's not really any worse with Vista I don't think. Not on new PCs with new equipment anyway.
 
The jumping sound thing - I'd sure get the latest drivers for my sound card and if that does not fix it I'd look into it more deeply. That sure sounds like a possible driver issue.
 
Could you list your system specs? How much RAM, sound card, motherboard etc. Thanks. We may be able to help you narrow this down some.

My system specs are;
 
Intel Q9450  
2GB RAM (800mhz CL4)  
Asus Maximums Formula motherboard (Sound card comes with motherboard)
EVGA 8800 GTS  
160GB (7200RPM)  
Corsair 620 Watt PSU
Windows Vista Ultimate 64 bit

 
You may need to reinstall the OS.  Have you run any kind of Registry repair or cleanup software on Vista 64?  Do you have all MS updates installed? Is SP1 installed?  Have you checked you mouse product page for updated drivers, if there are any for Vista 64?

 
Ouch!  Uninstall ASUS AI suite immediately.  There is a well documented problem with ASUS AI suite on the ASUS download/Forum page.  I have had the board for many months and had all kinds of problems when I first installed it.  I learned the hard way.  My mouse and keyboard would not even work.  Uninstall Ai Suite pronto.

 
I would download all the latest and greatest drivers from the manufacturers website.  They are probably newer then the ones provided on the disk.  

No clean up software. I'm sure all updates are installed. SP1 is installed. No I did not check for the mouse i'm just using the plug and play drivers.
Ok thanks for the heads up about AI suite.  
I know my nvidia drivers are up to date i'll check the motherboard now.

Yeah I never heard of Asus AI having a Vista 64 bit version.   My mobo is allot older but there were no Asus overclocking software for Vista 64 bit.  I would only overclock in Bios, if you have to, in Vista 64 bit.

I don't really need it because I can't overclock anyway. Not with the Q9450 running at 40C idle stock.