We use McAfee Asap, it doesn't use much individual computer resources (rumor-based) and the clients get the updates automatically from McAfee.com web site (or each other.)
As an Admin, you log onto the McAfee web site and enter your password to review what all the machines are doing, last time they updated, last time one of the scanners found a virus, and what happened to it. It has reports for viruses found each month, virus type, and which machines are being infected.
We installed this after Nimda and it has been super. Only snag is it doesn't work with another McAfee product, Groupshield, which was their previous mail server AV product (they have come out with a new managed mail service, so Asap seems to have removed some vital components as it automatically removes any other AV products when installed.)