*UPDATE*
Virus software removes virus's and things once they are on your computer. Which is why you scan your pc with it - to find stuff to remove. As a guide, you should scan your whole pc once a week and scan everything you download off the web before using it.
Firewall software attemps to block bad things getting into your pc in the first place so there are no virus's etc to remove
< Your firewall is the bouncer at the pub - Your virus software is your mate who comes looking for you to take you home >
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*Only install ONE instance of each type of security software - one virus and one firewall. Otherwise the programs will clash and be effectively useless*
I use AVG 8.0 which is completely free. It is (in my own opinion) the best free virus software out there. You can download it here - http://free.avg.com/ww.download?prd=afe
It is rated at no.1 out of 291 (free) anti virus products on http://www.download.com
You should also make sure you have some firewall software installed on your computer to keep the bad stuff out in the first place. I always recommend Comodo Firewall Pro which is also free. You can download it here - http://www.personalfirewall.comodo.com/download_firewall.html
It is rated at no.2 out of 112 (free) firewall products on http://www.download.com
Hope this helps!
:-)