Help With Outlook Web Access and Exchange 2003

I have a Win 2003 server with Exchange 2003 & ISA 2004 on one box. This has worked perfectly until a so called “MCSE” arrived to install an anti-virus solution on the server. Since then, we can access e-mails from Outlook 2003 internally, but internal clients cannot access Outlook Web Access. Can anyone tell me how to correct this?

Thanks
Brian