I've got some advice:
Use Firefox with Adblock and Noscript.
Most of the threat vectors these days are email and web based, and FF + the extensions I listed should protect from the vast majority of web based ones.
If you run windows, you need to have at least Windows XP SP2, and turn the windows firewall on. I don't see any need for an add-on firewall, the one included is quite serviceable.
As far as antivirus goes:
Trend Micro: use only if you have been recently thinking, man, my computer is just waay too fast, and I need to slow it down. Waaaaaay down.
Symantec: okay, and I've heard that the latest version (9?) is much less obtrusive (good thing!)
McAfee: okay
AVG: seems to not be too performance draining, plus it is free for non-commercial use.