Can clients be logged off automatically?

[Wanabe]

I have a couple of users that have a habit of leaving files open and not logging off of their computer. Some of these users use 98 and others XP/2000.

Naturally this prevents some files from being backed up at night and creates a security risk.

I don't want to enforce logon/logoff times because they do need to access the network from these workstations during off hours on occasion.

Instead, it would be very nice if there were a way for the workstation to automatically close files and log off of the network when a certain threshold of inactivity is reached.

Any ideas?
Thanks,
Wanabe