How to add computer to domains with Active Directory

Solution 1:

1. Right click on 'My Computer' and go to properties.
   
2. Go to the 'Computer Name' tab and click change
3. Change the 'Member Of' to Domain, and enter your domain name
4. You will be prompted for a user (AD Domain user) with rights that allow you to join.
5. Profit

Solution 2:

For Windows XP Pro: http://support.microsoft.com/kb/295017

For Vista business: http://support.microsoft.com/kb/295017

Windows 7 is pretty much the same.

You'll not be able to join any home version of Windows to an active directory domain. Furthermore, those local user accounts will still be available to log into. You'll have to change the local user accounts' passwords to prevent someone from logging into them. Also remember to migrate user data from a local account to the domain account if that's what you want to do. Use the free "User State Migration Tool" for that. Read all documentation. Your mileage my vary.