This is something i have had alot of experience with over the years with my own pc and many client's pcs. When most people go and buy their pc from a store they are generally sold anti-virus software with the pc. My experience has been that the majority of these problems are shiite. People will argue but it cost me $XX so it must be good and it hasn't found any issues. Well mos of the time this isn't the case, some anti-virus programs will basically scan right over the virus or trojan etc without picking them up. And the majority of anti-virus software out there does not detect spyware or malware.
I've come up with 3 simple to use programs that can keep your pc clean as a whistle.

http://www.eset.com/

Eset Security Centre is an anti-virus program that has a firewall incorporated into it

http://www.lavasoft.com/products/ad_aware_free.php

Adaware this is a comprehensive utility for the removal of malware and spyware. Plus it is free

http://www.safer-networking.org/en/index.html

Spybot Search & Destroy is another comprehensive utility for the removal of spyware and adaware. Plus it is free as well

Now all these programs combined can ensure that you are free from those unwanted nastys that you may pick up.

Young... er Older Padawan has learned well

I use Kaspersky Internet Security 2010 and Spybot Search & Destroy

http://kaspersky.co.uk/
http://www.safer-networking.org/

Oh, and I DON'T use IE! Firefox FTW!
http://getfirefox.com

Dont forget to mention all your lovely apps for firefox (NoScipt, NoPopUp, Etc...)

*cough cough* eset rocks !!