[cvsnt] Authentication protocol rejected access

[IAB]

I have installed CVSNT in a server and, when I try to connect it (with 
TortoiseCVS), I ever get the classical error: Authentication Protocol 
Rejected Access.

The server has Win2K, and the client has WinXP. I'm using SSPI protocol. 
Both computers are login in a domain (in other machine). The CVSROOT that 
TortoiseCVS is using is: CVSROOT=:sspi:user_name at ip_server:2401/repository 
where user_name is the client user in domain (I dont write it, but Tortoise 
get it) and ip_server is the ip address of the server ;-).

In the server, CVSNT is running and the port is 2401. The repository is 
'repository' and it is in d:\CVSRepo\Repository.

Times ago, I installed the same config. to test CVS and it works without 
many problems. I uninstalled all. Recently I installed again to really use, 
but  I'm blocked in the first step.

Which things can cause my headache? What can I do to test the correct work 
of each part?

Thanks in Advance.
IAB.