Desktop goes black after switching users

I have a Gateway computer with a Gateway 21″ wide screen monitor. There are only two users on this PC. Often times when switching from one user desktop to another user “Owner” desktop the monitor goes black and we have to do a hard reboot to get the system back up. I have tried unpluging the monitor and that does not fix the problem. The system is running Windows XP Media Center, has 2GB RAM, Norton Internet Security Pro, nVidia GeForce 7300 LE video card