I just wanted to say Phat has been enormously helpful.
The things he's told me to try haven't worked so far, but it's my computer's fault and he has done his best.
Also, for anyone who downloaded that Trojan Guarder that I suggested to several people: it was free for 30 uses, and I appear to have used mine up. But when I tried to close the screen that asked me to register, my computer froze up. I just restarted it twice, and had to do the alt-control-delete thing to keep it from launching itself again. Then I uninstalled it. Just a tip in case anyone's had trouble.