Lock computers

have a Server 2003 (full version) domain that I would like to create a GPO for.

I require that all WInXP Pro clients in the domain “lock” after 20 minutes of inactivity – and – that all workstations in the domain auto-reboot each night.
I do not want the idle time to log them off. I need their apps to stay open. The PCs must “lock”.

I do not know if the reboot is possible using a GPO. Anyone know?