My computers are losing their minds

I’m not sure what the heck is going on with my computers. Maybe I’ve entered some kind of vortex where there are random electromagnetic fluctuations traveling through the universe.
My primary PC is an Athlon 2500+ running WinXP. About two weeks ago it began randomly locking up. I assumed at first that it was the new memory I’d installed, so I pulled that out and brought it back to its former state. That didn’t solve the problem. So then I did extensive virus checking (I had recently been working on my neighbor’s virus-ridden machine.) That didn’t do the trick. After getting incredibly frustrated with my machine crashing in the middle of a multiplayer game of Rise of Nations, I decided to format the partition and re-install the OS. Okay, I don’t think that’s working either. WTF? I ran memtest86 and the memory seems fine. Is my motherboard losing its mind?
During the process of working on the machine today, I checked in with my Linux server and discovered that it too had crashed. WTF again. That’s the machine this site is hosted on. So I rebooted, checked the log files etc, and now it APPEARS that everything is fine. We’ll see if it locks up again. Confusion.
Guess I’ll keep hacking on it all. Might end up being the weekend project. Argh.