Group Policy

Hi
Our domain is currently in mixed mode NT4 and 2000
Every time I make a change in the default domain policy an xp pc somewhere on the network has a automatic reboot twice and then all is fine again. It does not save the users session. GP does apply to all xp pc’s. The GP is still very basic since we will only start placing systems in OU’s and setting up GP for that OU once we go native. In the event logs I receve an error :The Group Policy client-side extension Security failed to execute. Please look for any errors reported earlier by that extension. Event ID1085
The pc’s effected is random. Some new and some older. Some XP SP1 others SP1A. All service packs on servers are up to date. Why is this ahappening ?