aelgen: Those symptoms sound remarkably familiar to me (no issues running Prime95, but random reboots during casual computer use and even immediately after entering Windows). You haven't answered my question, so I wonder if perhaps you missed it: are you running any SATA optical drives or perhaps RAID? Additionally, did you set Windows to not automatically restart upon system failure?

I'm really hoping you answer yes to the former and no to the latter, since then the solution could be really easy. You mention Vista64, though, so I'm not sure if it would work for you, but it's possible.

Leeroy and loonym: I'm going to hold off on RMA'ing my PSU until after I do some more testing. I was reading up on PSU troubleshooting when it hit me that it could be my power strip. I'm currently forced to use power strips because there are no outlets close enough to my computer area. I may have had my computer plugged into a faulty power strip socket. I recall that I may have had some intermittent trouble with the same socket that the computer was plugged into when I was using the strip in my dorm way back when, but it was minor trouble and only happened once or twice, so it didn't come to mind. Why it would have waited a week to start acting up like this is beyond me, but I've got the computer connected to another socket right now and played Crysis for a while without any issue. The stuff I've seen would definitely be consistent (and more sensible) with a faulty socket.