You can also use this to test out the integrity of the machine to isolate whether it's a hardware or software issue. You don't have to actually install Ubuntu, you can just boot it off the LiveCD. You can also run memtest86 if you think it could be an issue with the memory chips (unlikely but possible).
This is a live Ubuntu Linux CD... just pop it in and boot off the CD. When you're done, shutdown and it will eject your drive so you can remove the CD and reboot to Windows.
The advantage of using something like Linux is that it is rock-solid and if it runs fine then you can at least eliminate the possibility of a hardware issue.
http://www.ubuntu.com/getubuntu/download