Minimize button closes programs

This is a Dell Optiplex GX 240 with Windows 98. It doesn't matter which program she's using, but when she clicks the minimize button on ANY window at all, it closes the program and nothing goes to the task bar. It's been going on for a few days. There is nothing in common about these programs. She's using Microsoft, Novell, Infotech, etc. Any ideas other than rebooting or scanning for viruses?