Exchange 5.5 Stnd on Windwows 2000

I am trying to put Exchange 5.5 on a Win2K server Sp1 on our production NT4 Domain and keep getting the message “this computer must be part of an NT domain before Microsoft Exchange can be installed” when I tested this install on our test AD domain I did not have a problem.
I have checked that the machine is on the domain and have no proble browsing and connectiong to the production domain
Does any one have any ideas ????? Please help

Andrew