Thanks for the input guys. I think I sussed out the problem.

At one point, I had attempted to put an Athlon XP 3200+ CPU in there, but the motherboard only supports just shy of that after firmware updates. I replaced it with the 2100+ CPU I'd originally removed from the computer and things run MUCH smoother (well, that it runs at all is an improvement). The CPU temps are reading in the high 20's, too, so that's great.

I was easily able to get Windows 7 to install. I'm repurposing the PC as an HTPC and want to try a bunch of OSs to see which one I like the best. I'll be dual- or tripple-booting it with Windows 7 for Media Center, Ubuntu with Boxee, and maybe Sage or MythTV. Any opinions on those options? I really think that MS has a great product in Media Center, and it would of course jive the best with the other computers on the network. I'm not above trying other options, though.