Free Virus Software, anti spyware & anti rootkit

AVG
http://free.grisoft.com/doc/29116/us/frt/0

Comodo
http://www.comodo.com/

Spybot is a free program that eradicates a lot of risks
http://www.pcworld.com/downloads/file/fid,22262-page,1-c,antispywaretools/description.html

Adaware is a free program that helps with privacy and stops spyware.
http://www.lavasoftusa.com/products/select_your_product.php

Avira AntiVir® PersonalEdition Classic
http://www.free-av.com/

spyware blaster
http://www.javacoolsoftware.com/spywareblaster.html

http://www.ccleaner.com/

CCleaner is a freeware system optimization, privacy and cleaning tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware!