Check server to see the user is logged in

[Kirubahar Sundarampillai]

I am trying to apply single sign-on method to our client server
application.
Our client server application normally use our own password scheme.
We want to see that the client is already logged into the particular
NT5 server
and if it is already, grant them to use the server application rather
than
use the our own password scheme.

I am new to this environment and already read so many article on
kerberos, sspi
and others. All lead me to a confusing state. Please someone help me
to point out to a right direction or sample codes.

Thanks
Kiru