For some reason, my desktop totally freezes up (except for the mouse)

after about 5 min of running. I took the hard drive out and hooked it up to my laptop and scanned it; no viruses. So I plugged it back in and it ran once w/o crashing, but now it freezes up again.

I was thinking it was a zonealarm problem; I removed ZA, but it still crashes.
Any I deas how to fix this?