We were hit hard by a new virus W32.Gaobot.WO, discovered a week ago by Symantec.
Long story short, it struck us before our virus program (etrust innoculateIT) has included the signiture on an update - meaning that about 90% of our 260+ computers were infected within a few hours.
The network traffic that it generated slowed the entire network down to a crawl, no-one could email, browse the web, run network programs etc...
http://securityresponse.symantec.com...gaobot.wo.html explains what it is and how to remove it manually.
It is now 3pm and all of the machines are turned off bar a few, so that we can remove the virus completely before turning them back on - as they reinfect very easily.
Companies in particular should keep an eye on this..