Programs with Real Time Protection:
Windows Defender - Free from Microsoft and now an integral part of Vista, Defender started off somewhat shakily but is now much improved. Comes with a full feature set and solid spyware-removal functionality along with a variety of scan options, including a scheduler. Active protection is enhanced by the inclusion of a heuristics component to help protect against unknown threats. Also includes a very good startup manager. (XP/Vista 32/64bit)
Spyware Terminator - Real-time protection monitors key spyware types and Windows objects, a snappy but simple interface uses buttons and tabs to navigate and set options. Updated spyware signatures are downloaded automatically helping to make ST a good set and forget solution. You can opt out of the included ‘extras’, just be careful what you accept during install. (XP/Vista 32/64bit)
Spyware Doctor (Starter Edition) - A limited, free version of the reputable Spyware Doctor from PC Tools. Includes basic real time protection, scan and remove, automatic updates and immunization feature. Even though Starter Edition’s cut down protection is an obvious ploy to entice users into buying the full version, it is still a useful free addition. (XP/Vista 32/64bit)
Others:
Spybot Search & Destroy - Somewhat limited protection via immunize feature and hosts file blocking. Not the best in this category but still a good choice second anti spyware. (XP/Vista 32/64bit)
Spyware Blaster - Mainly focused on Internet Explorer. Blocks ActiveX based attacks and spyware delivered via web browser. Useful subsidiary, particularly for Internet Explorer users. (XP/Vista 32/64bit)