Client PC turns on when shut down

I am using Windows Server 2003 R2 as a Domain Controller and running on XP SP3 client PC's.

My colleagues has complained to me that they are shutting down the computers every day when they go home. but it is already on for login screen when they come to office next day. Looks like the computers starting itself after 10 - 20 minutes.

I haven't done anything in the Domain Controller / Group policy. but i am installing all the critical updates using WSUS.

Does anyone knows what could be the issue?

Is there any setting can be changed in the Group policy level to fix it?

Thank you

Shaheed