Adaware and spybot are my anti-spyware programs of choice, a decent free antivirus is AVG.
Its unlikely to be a virus thats causing the problems, would definately guess it to be spyware.
Also, when u run the antispyware/virus, run them with your computer in safe mode, otherwise they may not be able to clear the problem as it is in use.
If that fails, google the spyware programs name, there should be some removal tools/guidelines for manually removing the correct files, processes and registry entries.
Also take a look at Macafee and symantec/norton websites, they can be pretty good for virus/spyware description and manual removal.
Hope this helps..