DS 6.9 SP6 - DOS PXE cannot map F:

I have a new installation of Altiris DS 6.9 SP6 running on Windows Server 2008 R2.  Linux and WinPE PXE works fine.  However DOS fails to map the F: drive giving a net use Error 53: The computer name specified in the network path cannot be located.

I know the DOS image is getting an IP address as I can manually map our old eXpress share on a 2003 server.  I'm thinking there is a difference between 2003 R2 and 2008 R2 that's stopping this connection.

Thank you for any help.