Anti-Spyware Software:
Spybot S&D is a free piece of software that can detect and protect your system from Adware and spyware.
http://www.safer-networking.org/en/index.html

Ad-Aware SE is another piece of freeware, which used in conjunction with Spybot, can provide an excellent rate of detection and remover.
http://www.download.com/Ad-Aware-SE-Personal-Edition/
3000-8022_4-10045910.html?part=dl-ad-aware&subj=dl&tag=top5

Grisoft's AVG suite is a free anti-virus software suite with regular virus definition upgrades. AVG is free for private, non-commercial, single-computer use.
http://free.grisoft.com/doc/avg-anti-virus-free/lng/us/tpl/v5