I posted a similar question in the PSU forum, but thanks to the help there I think I've narrowed down what may be wrong with my computer. It's 'homebuilt' by a small computer store from my hometown. I'm 900 miles away, so taking it in isn't an option.
Story: I decided to upgrade my PC. I ordered more RAM and a video card; the video card arrived yesterday. I hooked it up, turned on the computer, and everything appeared to be working fine...until I realized my PS/2 keyboard didn't work. I tried using a borrowed USB keyboard to no avail, but after letting my system sit for an hour it suddenly worked again. Thinking the problem was solved, I tried the video card and wound up with the same keyboard issue (which hasn't gone away for almost 24 hours).
It turns out that my PSU, though "rated" at 450W, actually carries 22A at 12V making it a glorified 300W that the members of this forum so appopriately called a doorstop. As part of my upgrade efforts, I put in an order to newegg for a new power supply earlier today.
Since then I've been investigating the motherboard and found a few inquiries online about broken keyboards. They recommended resetting the bios, so I looked up how to do that without a keyboard. I tried taking out the watch battery, and then switching around connector on the three pins beneath it. I just tried it and found that nothing was working.
Does it sound like I blew the motherboard when I put in the 9800 GT video card? According to power estimate sites, my system drew 280W without the card, and close to 400W with it. Would this be enough to damage the motherboard and, say, destroy the functionality of keyboards?