Archiving Exchange 2000 Data

Looking for suggestions to following:
I administer a Exchange 2000 Server.
The store is nearly 16 Gigs which is the max size.
I have asked users to “clean” their mailboxes but have not made much progress.

I would like to implement and archiving scheme which would archive mail out of their boxes on a regular bases without their interaction. It is extremely important that they can access these emails afterwards. This should be a server based archive tool. PSTs are somewhat ackward to use and can corupted faily easily. I will use them if absolutely required.