Both of the two 'big names' in security software, Norton and McAfee, have a reputation for being 'resource hungry'. (i.e. they can slow your computer down). Repeated tests by various PC magazines, as well as BBC News 24's 'Click' programme and the BBC World Service technology programme, have shown that much of the free software is not only as good as the commercial stuff, it's often better.
Your suggestion of using AVG 7.5 Free Edition is a good one. It's certainly the most popular choice of program with the 'regulars', here in AB's Technology section:
http://free.grisoft.com/freeweb.php/doc/5390/l ng/us/tpl/v5#avg-anti-virus-free
Most people pair it up with Zone Alarm Free, to provide their firewall protection:
http://www.zonealarm.com/store/content/company /products/znalm/freeDownload.jsp
(NB: Use the bottom link. The upper one downloads a time-limited trial product).
These are frequently 'topped up' with anti-spyware programs, such as Ad-Aware SE Personal:
http://www.lavasoftusa.com/products/ad-aware_s e_personal.php
and/or Spybot Search and Destroy:
http://www.safer-networking.org/en/index.html
NB: You shouldn't install any non-Norton software onto your PC until you've removed Norton. You can't uninstall it from 'Add/Remove Programs'. You have to download the 'Norton uninstall tool' from Symantec:
http://service1.symantec.com/SUPPORT/tsgeninfo .nsf/docid/2005033108162039<