Another vote for AVG Free. Another vote for Norton Spoiler:
can go fuck itself. Other than AVG, many of my friends also use
Avira AntiVir without problems.
There is
filehippo.com that provides a nice list of anti-virus software that they test themselves. Also remember that an anti-virus is not all you need once norton is gone.
Anti-Virus
Firewalls and Security
Anti-Spyware
I keep an updated list of freeware that I personally endorse in the
The Open Source/Freeware/Gratis Software Thread (
my list linked here).