Computer Keeps Locking, Freezing, Etc

Allow me to backtrack. A bit over a month ago, my video card started to show some pretty obvious signs of nearing demise. Graphical glitches in video games, random screen black-outs, and even more random total computer lock ups--but only when playing games. So, I replaced it. All was well. Until a couple of weeks ago. Now, all I'm getting is the lock-ups. These stopped when I first replaced the card, and only recently started happening. These locks are...different, though. For starters, they happen whether I'm playing a game or not. I don't have to be doing anything even remotely graphically intensive. Just staring at my desktop after startup will occasionally freeze. That said, it happens most often while I am playing games--but a counter to that would be that most of the time I'm on my computer, I'm doing that. So it's not exactly a reliable conclusion.

Something else strange about these freezes is that...they usually come to a slow halt, rather than slamming on the breaks. It's most noticeable when I'm typing. Letters will show up a split second late and within a few seconds, the computer is a brick until I manually reboot it. I've tried a few things. The video card is seated properly, the drivers are up to date. I cleaned out what little dust was in my computer. I defragmented. I tried to reinstall Vista. Tried to. I pop the disk in and it does its thing for what feels like forever, only to give me an error message and revert my system back to before it started. Honestly, I didn't write the error down though I know I should have. It was dawn, and I was too busy slamming my head against my desk.