AVG, if your into pirating get kaspersky though, it has just about the lowest memory footprint, and updates faster than the rest(were on botting forum figured i'm allowed to talk about piracy...)
Norton IS - Useless, also resource Hog Avast - good for freeware but does delete any files that has viruses in (deleted alot of system32 files) but that was along time ago when i used it. AVG - also good choice for freeware but it does slow your pc down alot on boot-up due to heavy update's (dont recommend if you have a old pc) Bitdefender IS - was my 1st choice in the past due to being a low resource hog but it had a low detection rate Avria - Not really tested it but had possitive feedback from fellow friend's for being good and low resource hog NOD32 - low resource hog, but i hated it could never get it to pick any viruses up not even well detected known keylogger's. Macafee - Also good choice but it does conflict alot with blocking alot of program's so you have to manually remove them from the blacklist. My top favourite all round choice would be Kaspersky Internet security 2010 due to having a low CPU usage and one of the best detection rates on the net, been using it for 2 year's and cant think of a negative problem i have had with it, apart from all my pirated key's getting blacklisted atleast twice a month lol. Also most of them program's could of got alot better since i last checked a year ago but not Norton it has always been bad lol Also the best way to keep safe is mix match program's such as a program for Spyware, Viruse's,malware,firewall etc.. check out Majorgeeks.com for review's on standalone program's and freeware.
Avast, free, well can be paid if you want more protection, uses low almost nothing of the computer and detects viruses well. Both the paid and free version works well.