Simple Solution (Score 1)

I have not had time to read every post, so I apologize if this has been mentioned before. I think putting up a gateway server based on Linux would be very helpful. A product like ClarkConnect (http://www.clarkconnect.com/info/) would serve well in that situation. This is the solution I have used. A quote from their website: "The award-winning Linux-based solution includes firewall and security tools, along with file, print, web, e-mail, proxy, antivirus, antispam, content filtering, VPN servers and more. A detailed feature list is shown in the sidebar below." You could also use something like SmoothWall (http://www.smoothwall.org/) as a gateway as well. By setting up a true firewall you can limit the outbound as well as the inbound ports. This will not eliminate the problem but reduce it to a great extent. Further services like anispam and antiviri will bring the number of issues down considerably. The downside to this is the the computer will have to be somewhat beefy. For 500 users look at 3.4Ghz with 2+GB ram and a few nic cards. I know there is no $ for this project, but that is just not reasonable. Getting the money for this would be VERY easy. Say the cost was $10K this would only be $20 per person (500 users). This could be charged as a one time fee or spread over a monthly payment (barring the school would front the $). You could also start this by charging users a fine for not following documented procedures for using the network. Uncontrolled Virus: $50 fine. Allowing your machine to be a Zombie Server: $150 Etc etc etc.. You would have the money in no time! Good luck with this, it may seem impossible now, but it is really not that hard to fix.