It may help to bring your computer up in "safe mode"....(on a windows machine you typically hit F8 as it boots up until it asks you what you want to do).
Then go to
http://securityresponse.symantec.com/avcenter/FxSasser.exe and download the sasser worm removal tool.
Run it and if it finds anything it will tell you.
After you get your system clean, reboot it then go to
http://windowsupdate.microsoft.com/ to download security updates to plug the holes exploited by the sasser worm.
Then I'd recommend you download a virus scanner and keep that up to date. If you don't want to pay anything for it, I'd recommend AVG,
http://free.grisoft.com/freeweb.php/doc/2/lng/us/tpl/v5. You'll have to download and install it, and then run the update wizard so your system is completely protected.
Once all of that is accomplished, make sure you virus scan your system. Most unprotected (or outdated systems) have many worms, viri, and trojan horses.