Random lock ups

Well my computer is fine (for those who read my post irql_not_less_or_equal). This time the problem is my girlfriends computer. Specs are as follows.

ASUS P5N-D motherboard
Intel q8300 CPU at 2.5GHz
Windows Media Center Edition SP3
nVidia GeForce 9800 GT
4gb of DDR2 memory, 2 gb ddr2 667, 2 gb ddr2 1066
600 WATT psu
200GB SATA HDD
60gb IDE HDD

Problem:

During normal activities, the computer will lock solid, no error messages, no event log, just solid lock up, only thing possible to do is power it off and back on.

What I've done so far:

Removed memory individually, problem persisted, ran CC cleaner. Ran Prime 95 to check for problems, ran for 20 minutes without any problems. CPU temp max 140, generally around 130. The only event log it leaves, is as follows:

Faulting application (app name).exe version 0.0.0.0 faulting module unknown, version 0.0.0.0, fault address 0x00000000.

Computer still crashes, generally it takes over an hour, temperatures seem fine, not quite sure what to try next, any ideas?

Thank you for taking the time to read this and for any help of any kind offered.

I assume those temps are Fahrenheit?

Try checking the hard drive for errors.
Start-->run-->chkdsk /r-->the scan will run the next time you restart.

Then check the system for errors.
start-->run-->cmd-->sfc /scannow

Good deal. Let me know if you need anything else.