bill - 8:32 am on Feb 28, 2005 (gmt 0)

A more powerful firewall like ZoneAlarm might be an easy first step.

There are all sorts of precautions that you could take to prevent this depending on your setup. Make sure none of the PCs are running with Administrator permissions to start with. You might just want to do a clean install of windows on all the machines and set up some security guidelines.