Make sure your listener is up and running.
you can verify if its running by executing the command
"tnsping D2KINST" where d2kinst is the
name of your instance, as defined in your listener file
and if it is unsuccessful you have a problem with your listener.

I don't know the name of the executable on NT but
locate the lsnrctl and execute it to start your listener(s).

Just a thought!