1. You get what you pay for
I'm not sure that adage is true w.r.t. anti-virus programs any more, Zydor. The big two have been relying on name-brand recognition for a very long time, but without really innovating, or, frankly, doing all that great of a job at finding viruses.
I'm a computer technician, and I can't tell you how often we get machines that have MacAffee or Norton installed, but, at the same time, are just riddled with viruses. (Possibly because the people who install those programs seem to thing that they're protected, but
never update the virus definitions or do a full scan.)
I use AVG, personally, and, while I find some things about it annoying as hell (stop force-updating when I'm playing a game, goddammit!) I think it's much better than the name-brands, and probably better. Some of the online scanners are also good, and can be useful for finding stuff that installed AV's cannot.
But, man, seriously, you should see some of the "protected" computers I have to fix sometimes... oy vey.