The university netreg lists has a frequently asked question if its
possible to perform a virus scan of new computers as part of the network
registration process. So far, people have only been able to do a network
scan (e.g. open ports), or some version of proxy check or nessus.
The University of Florida has implemented something like this.
Apparently, they have a client-side app that detects malware...and P2P
apps. Interesting concept but it's understandably not being received well.