Where did this domain come from

When I go into Entire Network and choose Microsoft Windows Network I see our domain plus another domain there which I have no idea who is it or how it got there. I click on the domain, than click on the server and a window pops up that is asking for a login and password.

I went to DNSSTUFF.com and entered the domain name into whoisit and nothing came up.

How do i find out who this is and how do i get them off of our MS Windows Network?

Thanks,