There is a risk factor to installing av on your exchange server as Lardum mentioned. I would suggest specifying which directories the av scans if you must have a av. In theroy, if you have you email AV upto date and manage (check out Norton's Corp edition...which can push virus defs) and/or lockdown your desktops then you shouldn't be worried about viruses on your Ex server.
Also check out Antigen for Exchange, by Sybari (sybari.com). I nixed my Norton's for Exchange for this and haven't had a virus make it through since. This product has excellent detection, repair and def updates. The is one of the more proactive virus companies I've seen. Mantislee
Systems Administrator
mantislee@yahoo.com