Here's what I have for protection on my computer and run regularly (all are freebies or trial versions), which may or may not be overkill, but makes me feel safer:
Zonealarm
MS builtin firewall
Ad-Aware
ewido security suite
Stinger
SpywareBlaster
SpywareGuard
Spybot S&D
Webroot Spy Sweeper
AVG
PestPatrol
CWShreddar
CCleaner
They all work well together, and other than some adware, I have never had an infection, BUT, I don't use this computer to surf the net. This is my trading computer, and so it is vitally important to me that it remain clean.
I have disabled most of the ActiveX and other controls in I.E., disabled Messenger and mostly use Mozilla Firefox as my browser (safer than I.E.). I use Mozilla Thunderbird, not Outlook Express, for email (again safer), and never open unsolicited emails.
Keep in mind when deciding what to use that generally one can only run one antivirus program at a time; for example, AVG and AntiVir, both good programs, conflict with one another, and cannot be run simultaneously. From what I understand Norton is a resource hog, and doesn't do as good a job as the above combination of programs, and they're free! Being freebies, most of these programs do not update automatically, so if you use them, don't forget to update each one regularly.
Good luck to you.
Newly