generally you should start up the computer in safe mode if using xp then do the virus scan using avg.
i have used nortons forever because that is what we use at work but there are some great ones out there.
another one is pest patrol which is from computer associates. avg is free!!
the virus will most probably be in your run folder in your registry.
if you are game then click start then run and type in regedit and press enter.
open the following folders in the sequence
there will be a weird file name, if not sure google the name
delete it only if you are sure.
this file will have been created with a couple of others and may be in your system32 folder in windows as most are and may even be in your program files as well. that is why you should find where they all are first then start up in safe mode and delete them. take note of the date it was created and do a search for files modified on that date. if you try to do this in normal running mode it will not allow it as they will be running processes.
sounds scary!!!! just get an anti virus and run it in safe mode, avg should work.
i dont expect you to do this all but it will help you if it happens again before it gets too far in your system. (IMG:style_emoticons/default/chin.gif