Idiot. -__- You pick one Antivirus and that's that. Change to another one if you must, but don't run multiple Antivirus programs at once.

Personally I use avast as my Antivirus. It's very good at not letting shit past it. Be forewarned, however, that the update alerts can scare the fucking shit out of you if you have the volume up high but aren't listening to anything. <_< Also, for the last few days it's been randomly using up 100% of mah CPU and slowing the computer like crazy. Pisses me off as the whole point of avast is that it's supposed to be very resource friendly and work in the background (which it used to be). Gonna see if wiping it and reinstalling will help (also bear in mind that my computer is a piece of shit that's dying so that may most likely is the cause and no fault of avast's).

Anyway, Avast, Avira AntiVir and AVG are the three best in most people's opinions. From the little I know, Avira is slightly better at detection of viruses, whilst Avast has the most features (no scheduled scans though <_<).

I use Ad-Aware for adware and PG2 (not sure if it has been ported to vista) and a good router as a firewall and they've done me well so far. Apart from that, the best defence is just common sense when using the computer (never run an executable from a source you don't know can be trusted etc.).