Do you have an Antivirus Application?
if you do (Good onya - congratulations) make sure it's virus signatures are up to date and do a thorough scan of your whole system.
If you don't have an Antivirus Application slap yourself around the head a bit at the same time as chanting "what sort of silly sausage am I?"
and then Download AVG free antivirus from here: AVG Free - Download antivirus and antispyware software for Windows XP and Vista
update it's signature files and let it do a thorough scan of your system. If it finds anything let it remove the virus or trojan and do another scan to make sure it has done it's job. It might be wise to run something like some sort of anti Spyware software like SpyBot available from here: The home of Spybot-S&D!
If that don't work and you've read all the links that Ursula posted for you, post back here and someone may have some more information for you.
Good Luck!