Click on Start > then right click on Computer and click on Properties
The basic system information page will open, under Computer name, domain, and workgroup settings, click on Change Settings
On the System Properties page, click on Change...
Under Member of, select the radio button beside Domain: , type the name of the domain you want your Windows 7 to join and then click OK
A pop up box requesting to enter credentials of an account with rights to join the domain (with rights to create computer accounts), example of such user is one that is member of the Domain Admins Active Directory group. Once the username and password are typed, click on OK
A confirmation pop up box will welcome you to the domain. Clicking on OK, will result in having another message informing that a reboot for the machine is required to apply the changes. Click on OK
Close System properties page, and on the You must restart your computer pop up message box, click on Restart Now
Once your machine is rebooted, press CTRL + ALT + Del to log on