There's no such thing as the best antivirus.
Most of them have similar virus-bases, none of them "know" all of the 0-day viruses.
Most of the antiviruses protect only againt antiviruses, so adware/trojans and other shit is still a problem.
No antivirus will protect you 100% of times against everything and the more defense modules an antivirus has the more of a hog it becomes, if you're paranoid and you do have a powerful computer that's your choice.
Personally I used Avast for years on XP, but with latest releases it became a hog, so I started using Comodo Internet Security (antivirus+firewall WITHOUT the Defense+ module which is also a hog).
Aside that, I use Firefox with adblock, Spyware Blaster and Spybot Search & Destroy to block all nasty sites, PeerBlock to block all nasty IP's from outside and Malware antibytes from time to time, just to scan C: if I'm clean.
PS: I'd worry more about that god-awful browser you're using, if I were you, walkie. Here
's an instant-crash link, that works only in IE as a example of it's "features".