I recommend Norton Antivirus Corporate Edition. They have licensing options that allow you a "per seat whatever software" method instead of having 10 copies at this price, 5 at that price, 2 at yet another, convoluted seat prices beyond.
It allows for centralized updating of all antivirus data, and they provide you with software that will install on workstations, servers, firewalls, mail servers, gateways, the works. You can download updates 1 time for all related machines and then push the updates to your clients, saving time and money in bandwidth usage charges and preventing user downtime and intervention requirements as the entire process can be automated.
Last time I checked NAV was the #1 solution. I know it consistently finds things other scanners miss - I get blips all the time for email attachments that have one worm or another stuck in them.