couple of points
currently free AVG is scoring lowest of all the virus scanners (84%) avira is just as free ... but more efficient at 92%.
Buenchico is right ... best include a firewall
zone alarm
the installer includes ZA and internet security ... so it is bigger than it used to be. but the windows XP firewall only stops incoming traffic .... Vista stops in and outgoing .... but does require a certain amount of knowledge.
zone alarm is in effect "plug and play" .... it's maybe not the best .... but it "learns" about specifics on YOUR machine and way of working .... so on that alone it's better than either window's version.
as for totally removing norton .... google your version of norton "remove norton 2006".
or go here
http://service1.symantec.com/SUPPORT/tsgeninfo .nsf/docid/2005033108162039
follow the link you require