I used to use Norton protection products until it became clear that they were making my computer run at half speed.
To regain performance I un-installed Norton and used the MacAffee "Internet Security", which was better apart from the fact that the anti-spam part of the product completely locked my machine for 10 minutes when e-mail came in, and then had quite a high false-alarm rate so I ended up checking e-mails by hand anyway.
So when the 12 month renewal premium cam in (and would cost me more than buying it new again at the local PC store) that went too.
Now I use the free version AVG Anti-virus from GriSoft and it hasn't let me down yet.
Spam is filtered using rules in Outlook; crude but effective. I'm thinking of changing Broadband provider so that will force a change of e-mail address which hopefully will stop a lot of **** for a while!
You may still feel the need for a Firewall. If your PC hooks up to the internet via a broadband router of some decription then you are already protected by a hardware firewall and so it can be argued that you don't need a software firewall at all. I'm not sure I'm totally happy with that argument, although at present I'm just using the standard "Windows XP SP2" firewall and it hasn't registered any naughty access attempts in 9 months so maybe the conjecture is correct!
Nevertheless "Comodo Personal Firewall" has been mentioned in a few GB PC Magazines as a pretty good system, and recommended to me at times, and it is also free.
http://www.comodo.com/