1) if you have WinXP or Vista, try System Restore to undo the damage. It may work, and it may not, but it's simple to try as a starting point.
2) if that doesn't work, then you might want to try
John's
malware-removal guide.
3) if you're still experiencing symptoms, one effective option is to nuke your Windows installation, reinstall Windows, and
secure it properly this time. Another option is to get expert help at a forum that specializes in malware removal, like the CastleCops forum, which may take some patience and discipline.