Personally, I prefer AVG Antivirus, free edition. 100% free, free updates. http://free.grisoft.com/
Zonealarm is good, I personally use one called PC tools firewall plus, both are available for free as far as I know.
Plus of course run windows defender and keep your pc updated. That's what I would run myself. I do some light gaming on mine in vista as well.
AMD Athlon 64Socket 939 3500+
1 gb ram
Vista business, 32 bit edition.
And yes it's due for an upgrade, but I actually do a little light gaming on it with those programs running in the background. So I think you'll be good.
Also, adaware and spybot search and destroy are good anti spyware programs, again there are free versions available. Go get the free stuff and buy yourself a nice game and enjoy that new PC:).