Start > Right-click Compter > Properties
Click Advanced System Settings
on the left
Go to the Computer Name tab
next to "Rename this computer etc"
Check the box for Domain
, not Workgroup. Type in your domain name and press ok.
You might have to enter the details of an Administrator account and you will have to restart the computer.
Failure to join the domain at this stage is commonly a DNS issue. Be sure your Windows 7 machine is using the 2003 Server as DNS and not your router. This is quite a common mistake.
Same process explained in this guide but with pictureshttp://helpdeskgeek.com/windows-7/windows-7-join-domain/