A friend had the exact same problem a couple of weeks ago. It's a virus that has been doing the rounds and has changed registry settings.
Look here for a similar virus analysis from the sophos website :-
http://www.sophos.com/virusinfo/analyses/trojfavaddc.html#table4Find out the name of the virus you had or run spybot-sd to see if it will pick up the registry changes.
Other method is to create a new user account and login with it but then you lose all your desktop settings, my document locations (these all need reset). Even then it may have infected the registry globally (all users subkey)