Time settings change randomly

I have a user whose Windows 2000 Pro. time settings change randomly, sometimes one hour ahead and sometimes one hour behind. Any idea what could cause this?

He does not switch between time zones, always uses the same one (CET), also AutomaticallyAdjust for Daylight Savings Time is selected.
Seems to happen randomly, not when connecting or disconnecting to network.

Also no recently installed applications seem to have caused this.
CMOS battery has been recently swapped.

Any ideas greatly appreciated.

thanks
Jimbotech