RPC and RestrictRemoteClient registry value in XP SP2

[Guest]

We have a RPC server application that registers with end point mapper.
But RPC clients aren't able to connect to the server in WIndows XP SP2
enviornment.
When RestrictRemoteClient registry is set to 0 it works fine. But by default
in a XP Sp2 environment this key is not present. [and hence default value is
1]
We tried suggestions in MS articles about using "RpcServerRegisterIfEx" and
security callback function with appropriate flags. But still the client fails
to connect with 'ACCESS_DENIED' error.
Any suggestions are appreciated.

Thanks
Krishna