Bitdefender is the fastest AV tool I have ever used. I have given up on Norton, to me Norton has become bloatware. NOD32 is also very good and is the least intrusive of the AV programs I have used.
FYI - Certain Windows Vista versions come with Bitdefender (Vista Ultimate, for example). You may want to look at the price of doing a live-update of your version (if you have Vista) to the upgraded version with Bitdefender.
Avast is no longer free, beyond a 30 day trial. They stopped the free virus scanning about 3 months back, I believe. They are good though.
I run Avast for anti-virus and Hitman Pro for anti-spyware and anti-malware. Hitman Pro is really an umbrella program for a TON of freeware and shareware anti-malware programs, but it's cleaned every infected system I've ever come across.