If you havn't already got the Sasser worm you will unless you get a patch.
The Sasser worm is not spread by email and an infected machine can scan up to 200 other machines for weaknesses per second. The worm has so far been found to be harmless (i.e. it won't wipe your HD) but it will continually restart your computer, sometimes so quickly that you won't be able to download the fix.
If it does re-boot to quickly for you to get the patch, click on START, then RUN and type command.com .when the command prompt appears type shutdown -a this will abort the shutdown.
Microsoft Windows update
Edit: Fixed the link.