w7 laptop-multiple probs w/W2K nw

Hello: these are thorny probs for me. The short answer (from youse) might be to steer me to set of (very simple) tutorials. Otherwise here's what I want to do:
-w7pro laptop to connect (cat 5e not wireless) to my home C/S w2K network, so that I can access files at a wXPproSp3 pc. I can connect to that same XPproSP3 PC from another XPproSP3 PC, and from my OS X.5 mac (using smb). I can connect from the w7 laptop to a shared file on the server.
I am not able to:
-rename the computer name. It is now known as 7laptop.[domain name]. The domain name is not the full domain such as xxx.yyy.gov. What the laptop calls itself is "7laptop.xxx" What if I was able to eliminate the xxx, would that simplify/fix the problem?
-not able to join the domain. The laptop is part of the "workgroup" called "workgroup ", but try as I might, I cannot seem to add it to the proper domain here in our home again as an example (not the true domain) "xxx.yyy.gov ". I keep getting "the active directory domain controller for the domain ( "xxx.yyy.gov ") could not be contacted ". So I can't change the computer name, nor join the domain, until I'm able to join the domain, a circular issue. There is only 1 domain controller, the w2K server, & everything seems fine there. Advice? Thank you.

 

Thanks Arie, but your answer didn't address the circular argument. I don't want the w7laptop in the workgroup. I simply can't ditch the "xxx" part of the laptop's so-called name and have it as just "7laptop." Anything I try on the laptop fails to change any settings on the laptop. I have also deleted the laptop from computers in active directory on the server, added it again-no change. Thank you.