You'll also need an anti-spyware product, such as Ad-Aware 2007 Free:
http://www.lavasoftusa.com/products/ad_aware_f ree.php
and / or Spybot Search & Destroy:
http://www.safer-networking.org/en/index.html
IMPORTANT: You should
never have two anti-virus products operating simultaneously. (It can cause serious system conflicts). Even though your Norton subscription may expire, some of its functions will continue operating. You
must fully remove Norton before installing either AVG or Avast. This can't be done from 'Add/Remove Programs'. You have to use the Norton Removal Tool, which you can download from here:
http://service1.symantec.com/SUPPORT/tsgeninfo .nsf/docid/2005033108162039
Similarly, you shouldn't have two firewalls running simultaneously. (Use Zone Alarm or Comodo, not both). However, because they're 'run on demand' programs (rather than 'run in background'), you can have both Ad-Aware and Spybot on your PC.
Chris