Standalone Virus Protection on SBS2003

Hi all,
I got a call today from a local sme who has just purchased 5 Norton Internet Security licenses, for their 4 PC’s and their SBS2003 server. The problem is NIS doesn’t support sbs2003 server.
So I’m looking for a good av package for the small business server. Now normally I would go with something like Trend or F-Secure, or even Symantec but I would be getting their sbs oackage that has the server plus a number of workstations.
In this case I’m just looking for a server AV solution. They’re running exchange, (using the pop3 connector) and obviously using it as a file server too.

Appreciate any suggestions
Cheers
T