Disappearing Windows 2000 Taskbar

The taskbar on one of the Windows2000 computers where I work disappears after 20 to 30 minutes. If the user logs off, and then logs back on, it will reappear. I have tried right clicking where the taskbar should be, refreshing the desktop, pressing Ctrl+Esc, and then right clicking where the taskbar should be to get the taskbar back, but nothing has worked yet. I will give 500 points to somebody who can help me.