I don't use one. I just use common sense.
Use a decent browser (anything but IE).
Don't open email attachments (but be careful if you do open any).
If I download software that's not from a trusted site I'll use an online scan. I used to use housecall
but since the version of java was updated it won't work for me any more so I found BitDefender
do an online scan so I use that one now.
In 9 years I've had 3 viruses, 1 was when I'd only had my pc 1 week (and it was a rubbish virus that didn't do anything) and the other 2 from a Japanese p2p prog. Oh, and I got a dialler from Robb's Celebs (that site was full of them) but I'm on broadband not connected to a phone line so that was another fail.