The one that i have used at work is Norton with the live update.
Never had any problems with it. As soon as there is a hint of a virus about, update the signatures on the servers and 1500 machines were protected in under 20mins.
The easest way of avoiding viruses in my experence is not to use microsoft products for reading emails and don't go anywhere near MSN unless you back your machine up.
And also the linux box which stripped vbs attachments off emails before they got near the exchange server might have helped as well.
MJ