|My first suggestion would be to test the memory.|
Bad memory can also corrupt system files. Go here and download memtest86. Burn it to CD and boot from it. Let the test run for several hours, in case there are thermal issues. http://www.memtest86.com/download.html
This might also be caused by overheating. It's not uncommon for the exhaust to become clogged with fluff.
Computers... designed to entertain,help with work but most of all to frustrate you!