Questions about security software come up almost daily.
There seems to be almost universal agreement that there's no need to use commercial software, such as Norton or McAfee. (Both of those programs have a reputation for being 'resource hungry' and slowing computers down). The free stuff is at least as good and (in most people's opinion) probably better.
Windows Defender is rarely mentioned but, the few references I've seen to it (both here and in the computer press) seem to rate it highly.
The vast majority of the 'regulars', here on AB's Technology section use a combination of Zone Alarm Free Edition (as their firewall) and AVG 7.5 Free (as their anti-virus solution). This is then topped up by the addition of anti-spyware software. The most popular programs are Ad-Aware SE Personal and SpyBot Search & Destroy.
Links:
Zone Alarm
http://www.zonealarm.com/store/content/company /products/znalm/freeDownload.jsp
(Ensure that you use the 'free' link, not the 'trial' one).
AVG
http://free.grisoft.com/freeweb.php/doc/2/
(Right-hand link)
Ad-Aware
http://www.lavasoftusa.com/products/ad-aware_s e_personal.php
SpyBot
http://www.spybot.info/en/download/index.html
Chris