NOD32 version 2.7 is an excellent one with a very low footprint (excellent for old computers for example).
Version 3 seems to be a ressource hog in all aspects, without providing any more signficiant safety (especially the suite, whose other components are not as polished as one may imagine).
Kaspersky 2009 (either AV or the full suite) can be an excellent alternative for easy allaround safety (the suite is superior to other packages in convenience and safety).
I tried the free version of Kaspersky they were doing alongside Windows 7 Beta, and it flagged Steam as a possible threat when I went to install it. Also, at one point, Windows 7 action center kept telling me a problem was identified that Anti-virus wasn't up to date, and I'd keep updating Kaspersky, so it went in a loop. Not sure if Windows or Kaspersky was the cause, but it persisted after a reboot. I think also got that same program for free with my motherboard, but I never tried or installed that one.
Admittedly, I don't use anti-virus software, so I can't speak outside of that. All I know beyond that is I've heard Norton is a resource hog.
shafeen, you're kidding right? ZoneAlarm is poison for a PC. I installed that a few months ago and it kept blocking my internet access randomly from time to time.....aside from bugging me all the time.
But yeah, Spybot S&D is pretty good. And Malware Bytes is also good if you want to run a scan once in a while (though you have to cough up if you want real-time protection). Nod32's personal firewall and anti-spyware is pretty crappy though.