Disabled Task Manager

My father is running XP Professional and when he tries to access the task manager from his user account he get's the following message

'TASK MANAGER HAS BEEN DISABELED BY YOUR ADMINISTRATOR'

I've looked in the system policy editor and the registry and cannot find anywhere to turn this back on. It used to work. I've also seached the help for 'disable task manager' and followed all of the recommendations.

Thanks, virtually yours,

William Main