Hey guys...
Thanks so much for your suggestions. The problem was apparently that I AM A WORTHLESS IDIOT and didn't notice that my mouse had run out of batteries! Sorry to wrongly accuse you guys, Stardock!
In my defense, though, the problem was exacerbated by the fact that my regular PS/2 mouse, which I plugged in as a backup, is apparently dead as a doornail as well. I've also updated my CPU and nVidia drivers, for whatever that's worth.
I am sure this is some kind of bad karma left over from the days when I used to do tech support. How embarrassing... Anyways, thanks again.