This is not a joke. Check your drives for any forgotten CD's or floppies, and remove them. Save anything you have been working on. Turn your computer off, using the officially-approved method. Remove the plug from the electrical socket. Go away for a while, at least half an hour, but preferably longer. Turn it all back on and see what happens.

This is the electronic method of giving your computer a purifying bath, and I've seen it fix some pretty strange problems. It can work for copy machines, too.