I have to say I was startled to see AVG Anti-Virus Free Edition. It worked better than an old version of Norton Anti-Virus. Also, you can find free Anti-Spyware and Anti-Rootkit software, and even Anti-Virus for Linux. If you can not afford an expensive but complete security solution, I guess the AVG free tools are just for you. You can download them here.
http://free.grisoft.com/doc/welcome/us/frt/0