One possibility...
Someone is resetting your computer.

>Hi,
>
>Today, my MS Window 2000 professional system is giving me problem of auto reboot when I manually lock the station and went to station for testing, on return it just auto reboot itself after around 10 minutes.
>
>However, if I am using it, I don't see it happen.
>I had done a virus scan and it is okay.
>
>Please kindly advise me what could be causing the problem.
>
>Thank you