Exchange prompts for authentication every 10 minutes

this is weird. NT 4 domain. 1 pdc, and 2 bdc's, one bdc is remote across 256k pipe. Exchange 5.5, SP4, 3 different servers with mailboxes in same site. None are DC's. Started happening about two weeks ago. I thought it was only happening to Office XP users, but know it is happening regularly to

AV on OWA Server

My OWA servers are stand-alone IIS servers (Exchange is not running on them). Does anyone know of a reason that would prevent me from running AV on them? I know this is a no-no with an Exchange server . . .. TIA Chris _ List