I think you may have the domain concept confused with regard to its use on a Windows 7 computer.
For a windows computer to "join" a domain, you need to first have a Windows server running Active Directory Domain Services. The domain concept in Windows refers to a centralized service that allows you to manage the authorization and authentication services for your security principals (users, computers, etc..).
The concept of a domain in Windows is not exactly what you are thinking regarding registering a domain name (DNS).
Not quite right JorgeM, I think! (Caveate Emptor).
I think the poster is referring to an internet domain, such as xyzzy.com. He has registered his domain (xyzzy.com) and wants it to be visible on the general internet, correct gerhardus01?
To do this, you need a static IP address from your ISP, and then go to your domain registrar to link the address to your domain name. Next, depending upon the services (web, email, etc) you want to allow into your "domain", you will need to allow access to the necessary ports through your firewall (router).
Finally, you will need REALLY good anti-virus and anti-malware tools on your server to make sure you don't get pwnd! :-)
So, move forward, and post more questions here when you need help on these issues.