A worm has infected over 3 million right away and may end up to 20 million PCs.  What seems to be the culprit is USB drives.  It's called either Down & UP, Down-a-Up, or Conficker.  It works by tricking you by running the virus when you plug in a drive. Look closely at the screenshot above and you'll see two entries for "Open folder to view files." The one at the top is a phony entry that actually installs the virus. When installed the virus spreads like crazy via a separate flaw in Windows networking system and can quickly infect a whole office.  Get your latest Windows Update to get that flaw patched up.

     To avoid this turn off "AutoPlay/AutoRun" on your computer and if you have it then go to F-Secure and that should work.