I chose not to have one.
I'm running XP sp3 and prettu much I know what I'm doing so why bother with something that is going to slow down the start up of the system and limit its performance ?
I use a good firewall yes ( COMODO, and it's free) and I browse the net with Firefox with added noscript and adblock plus.
I couldn't feel safer.
Of course I don't go around downloading dubious roms or fake txt files with hidden extensions like .pdf or .exe from unknown sources off porn sites for example, that would be outright stupid.
It's been four years since I decided not to have an antivirus on my PC, and I never had a problem, not even once.
I even run a couple of malware and spyware tests every now and then ( before somebody could claim that for sure I have viruses I'm not aware of ), and I've always come out clean.
Am I smarter then the average PC user ?