Win98 Start Menu Problem

I support a Packard Bell P120 w/48MB RAM running Win98. I can’t get navigate anywhere by going to the start menu. All the menus pop up but nothing happens when you click on any of them. Nothing has changed on the system, and I’ve scanned it for viruses with up to date definitions. The user said the problem began all of a sudden in the middle of the day and he had done nothing out of the ordinary. The icons on the desktop all work, one of which is Windows Explorer. I can get to all the programs through Windows Explorer. Except for not being able to access anything through the start menus everything else is working O.K. Ideas, anyone?