Antivirus software isn't enough. You also need good adware/spyware removers, which most A/V software doesn't handle properly.
Two good free spyware removers are AdAware and Spybot Search and destory. You can find them here:
AdAware Home Page
Spybot Search and Destroy
Each is good, and each one finds things the other one doesn't.
Adaware also offers a pay-for version for those who want a little bit of extra protection. Spybot exists because of user donations, and a contribution of $5 to $10 (via Paypal) will be appreciated (I'm not associated with them, but I do believe in donationware authors being compensated), but isn't mandatory and gains no privileges or extra features.