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...
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..