I my quest to find out what on earth is causing random lockups, I'm starting to think it was the power supply. The reason is that the problems started soon after I put my graphics card (old Radeon X800) and the demand for power went up. Recently I've kept one monitor disconnected (was a dual monitor setup) and it hasn't locked up at all.
Before you ask about the card, the lockups don't just occur when doing graphics heavy stuff like gaming, its during normal but heavy operation. I could be viewing flash, encoding video's, fragmenting, running a virus scan, remote desktop with semi-heavy application running, or other things that don't use the card.
Unfortunately I can't tell the exact wattage of my PSU. It seems this case (old Emachines W2060) used several different power supplies and didn't list this information on the PSU itself. There's 250w (although its a stretch), 300w, and 480w PSU's available, but I'm not sure which one it is.
So my question: Could an underpowered power supply cause lockups? This would mainly be under heavy load but could also be randomly. This would happen after anywhere from an hour to 6 hours of use.