I need help! my computer locks up completely to the point where i have to hard reboot. I tried tweaking with every option i know but i can't tell what's making it lockup. I tried reformatting the HD, reinstalling windows and all the latest critical updates, flashed the BIOS with the latest version, updated all video,audio,etc drivers and nothing it crashes mostly when i play video games but most programs will eventually crash including IE. I know its not the memory because i ran memtest86 for almost 15 hrs with no erros, and prime95 for a few hours without errors. The video card is fine too, i test it on another computer for hours with no lock ups or crashes. what COULD it be? are there any diagnosis programs i could use to pin point the problem? any help will be very much appreciated.

i'm running:
Asus P5N32-SLI SE Deluxe MoBo
CPU Intel X6800 @2.93GHz
Nvidia 7950GX2 vid card
2gig RAM Crucial Mem (4x512MB DDR2)
750W PSU
Cooler MASTER water cooled for CPU
Western Digital Raptor 74Gig 10,000RPM HD
Windows XP Media Center Edition 2005You might want to try the 'divide and conquer' technique...

-substitute a different video card or hard drive, then reinstall windows and let it run for a while.
-I've had systems lock up with bad ribbon cables to my hard drives, try substituting those.Also make sure you're getting airflow through the PC, your heatsink is PROPERLY seated, etc... Lockups like that can be caused by overheating.As of today I've monitored the CPU and GPU TEMPS. and they are within the average numbers. I've basically taken the whole pc apart to check for damaged cables but they all seem in really good shape so that's out of the picture. The only thing i haven't tried yet is swapping the HD, but I doubt that can be the problem. I mean its not making any noises to indicate its about to fail and after defragging it there wasn't any errors found. I really don't know what to do anymore and I would hate to take it to geeksquad for a 200 buck "check up". Any advice would be extremely appreciated , thanks.DLoad and run the diagnostics from the drive manuf. site...
DLoad and run MemTest for the RAM...let it run a few hours....
Any errors either way will point you in the right direction.I d/led the diagnosis software from western digital and i did the SMART test and the Extended Test and the HD is fine no errors at all. I ran memTest-86 again for a couple of hours and nothing either...
My CPU Temp. at Idle 35C and at full load 45c and my GPU runs at 60C idle and 82C at full load, is that average temps.?
Could my MoBo be the cause? and is there a way to diagnose?