Why don't you run a virus scanner that updates daily? Viruses should be no problem then.
Qive them a disk quota maybe that is small, so they don't DL a lot, and make sure that each user can only download to one specific place.
Also, what we do at work is scan the network drive people download to and automatically delete mp3s and other frivolous files (unless they talk to us first abvout needing the stuff for work).