AVG and avast are the 2 most popular
ones in here plus you need adware/spyware protection not to mention a firewall. Here are some of the recommended adware/spware programs that are free.
Spybot S&D (download, check for updates, read the tutorial and scan often, it also does some blocking)
http://www.safer-networking.org/en/home/index.html
SpywareBlaster (a blocker only, download it, check for updates, enable it and leave it alone except for checking for updates occasionally)
http://www.javacoolsoftware.com/spywareblaster.html
SpywareGuard (similiar to SpywareBlaster but works in a different way and does not update as often for that reason).
http://www.javacoolsoftware.com/spywareguard.html
Ad-Aware SE (a scanner, download, check for updates, read the directions and scan.
http://www.lavasoftusa.com/
cwshredder (stand alone unit)(another small scanner for certain things, ALL other windows should be closed)
http://www.intermute.com/spysubtract/cwshredder_download.html