I figured out what the problem was. Earlier today I ran Windows Update. One of the updates was a driver update for my graphics card, which failed to install. I was finally able to track down the driver online. I installed it manually and everything works normally again.
I've been caught in that trap myself. Back when I was still on dial-up, I let Update install a modem driver. Now I always check and de-select any drivers in the list. Then I choose the "don't show me this damn thing again" option.
Since your problem is solved, I'll close this.