Hurry and go to the Command prompt from the Run menu. (CMD in Windows XP/2000, COMMAND in 95/98)

Type "shutdown -a" to abort the shutdown.

Anyway, that sounds a lot like the ol' MSBLAST/LOVESAN worm. After you've aborted the shutdown, turn off system restore, then download/run this:

ftp://ftp.f-secure.com/anti-virus/tools/f-lovsan.zip

After that, get the RPC patch from Microsoft. (Just go into IE and go to Tools->Windows Update and let it download stuff. If you actually haven't updated, you'll have a LOT to download..)