Depends how deep it has penetrated your operating system - at work we tend to rebuild (re-install) the PC from scratch to wipe it out for good.
You may be able to clean it up by running something like Spybot s&d and then a scan with a good anti-virus program (make sure both are updated and then run them both in Windows safe mode). You may have to run them more than once. A registry clean with ccleaner may also help.
But in all honesty a rebuild may be the only lasting option..