Problems with random reboot

I built this system almost a year ago, and it's been running like a champ the whole time. My issue started about a month ago or so when I install Vista on my machine (it was previously running XP). I started having this weird issue where every once in a while, my computer would crash randomly. I passed it off as some weird quirk having to do with an incompatibility with my computer and Vista. Anyway, after a while, I couldn't really stand Vista anymore (had some weird issues, and I like XP better), so I popped XP back on as my main OS. That's when the problems start.

So now it's come down to my computer randomly reboots ever 30 minutes to three hours, depending on if I'm doing anything on it or not. I'll give you examples.

Example 1: I leave my computer on all the time. I like being able to turn on my monitor and jumping right into whatever I need to do. I was on my computer, left to go to the store for 15 minutes, come back in, and I see the screen that says that my system has just recovered from a serious error, and wants to know if I want to send an error report to Microsoft. I had a torrent client running and that was it.

Example 2: I'll have several windows open (Firefox, Adobe Reader 9, a messaging client, a torrent client, and a few other programs open for good measure) and my computer will, without any kind of warning at all, go black screen and I'll hear my hard drive spin up. Then the Windows logo will appear.

All in all, this is extremely frustrating, especially considering what I put into this machine. It has a 3.2 GHz dual core AMD processor and 4 gigs of ram. I've never had any problems at all running multiple programs for extended periods of time, and now this. I have no idea what's wrong with it, so if anyone has any insight as to what the problem could be, I'd be happy to hear it.

Thanks in advance.