I know there is a GPO for that when a user is using Terminal Services, you can even let the GPO decide to log the user off after xxx minutes, it monitors the last input given by the user. Not sure if this can be done on a client computer though...

[edit]
I've found this, hope it might help you in the mean time
Idle Time dll
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Edited by apronk (2006-05-05 08:38 AM)