computer asking to log off

Workstation running w2k, office xp all of the latest service packs and updates installed. Network running 2000 server. Network of 50 + workstations only one workstation having the following problem: workstation is periodically asking the user if they want to log off. Checked for viruses and it’s clean, asked other IT person if they were sending alert messages to “mess” with the user – no they aren’t. I have never seen this before looks like a microsoft message. Anyone have any ideas???