[cvsnt] SSPI authentication failed

[Raheel]

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Hello, I was using pserver before, but now I want to switch to sspi for better security. 
I defined my cvsroot like:

CVSROOT = :sspi:<ip_address>:/Test

F:\CVSWorking>cvs login
Logging in to :sspi:raheel@<ip_address>:2401:/Test
CVS password: ******
cvs [login aborted]: Authentication protocol rejected access

Environment:
Server:
    CVSNT 2.0.48
    Window NT 4.0 workstation

Client:
    Concurrent Versions System (CVSNT) 2.0.51a (client/server)
    Windows 2000 Professional

Note that user 'raheel' is in administrators group on NT 4 workstation.

Any expert thoughts over it??