Outlook 2007 with Vista running RPC Over HTTP

Hi,

Currently running SBS 2003, Exchange 2003, ISA 2004. Everything is setup and running well using Outlook 2003 Clients on XP Pro Workstations using RPC over HTTP. Recently updated workstations now running Outlook 2007 with Vista Business are giving me a Certificate Error when the user name and password is entered. “There is a problem with the proxy server’s security certificate. The security certificate is not form a trusted certifying authority. Outlook is unable to connect to the proxy server (Error Code 8) I have tried exporting the certificate’s from the server, then manually installing them, the certificate simply do not want to install. HELP!! Mark.