computer rebooting for no apparent reason

[ny_driver]

ABIT AV8 K8T800pro
athlon64 3500+
2x1024 supertalent ddr400
HIS 9600pro family 256mb
antec 420w PSU
acer 22" LCD monitor model p224w
windows xp pro sp3 fully updated

My moms computer just reboots itself occasionally. Not during any one particular activity, just whenever. She doesn't do anything real graphically intense and doesn't really do much multi-tasking either. Because her old computer also did this rebooting thing, I have suspicions about the monitor. It could also be the PSU, although I replaced that (with a used one) when I built her new rig. I replaced everything except HD, CD-drive, and floppy-drive. I suppose it's possible that the old PSU was faulty, and the new used one is also faulty in the same manner.

I'd love to hear some other ideas. Thanks.

 

[trents]

Have you checked the ram with Memtest 86+? It's a free diagnostic program that pros use. Download it from the net. It runs from a bootable floppy or CD. Remove one memory module and test one at a time. Run it for 3 or 4 complete passes. Takes about 45 min. to complete a pass. Often, if there is a problem it will show up on the first pass. The problem you describe is not likely a monitor problem. Monitor problems don't restart computers.