I use Avira now on my vista machine.
I used to use avast at one time, then I tested AVG, but decided to test other antivirus programs that were free. I heard that Avira has highest detection rates, Avast is most aggressive in scanning/protection, but AVG is best all around. That's what I heard, but who knows.
As long as all of them are free and all of them update or at least checks for updates at least once a day, I'd just pick the one that runs most smoothly on your computer (doesn't lag your computer) and has a GUI interface that you are comfortable with.