Spybot S&D is good, and I favor Avira for antivirus, highest detect rate of any antivirus software, there is a free version available funded by purchased versions.

Be careful to only download from the company website or a trusted repository like download.com or tucows.com, many sound-alike programs exist which are actually spyware themselves.

Or do what I did. Install Linux Mint 7 and forget about antivirus and spyware protection.