Click the start orb, and right click computer and choose properties
Click the Change settings link (middle on the right)
Computer Name tab
Change button click
Domain radio button
Type in the name of the domain (yourdomain.whatever)
you'll be prompted for domain credentials to support the joining of the domain, enter the domain admin's credentials
If the DNS is configured properly you should get a welcome and a prompt to reboot.
Reboot and log on with your normal domain credentials.