To join a domain
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
Open System in Control Panel.
On the Computer Name tab, click Change.
Under Member of, click Domain, type the name of the domain you want to join, and then click OK.
You will be prompted to provide a user name and user password to join the computer to the domain.
Click OK to close the System Properties dialog box.
You will be prompted to restart your computer to apply your changes.
To open System, click Start, click Control Panel, click Performance and Maintenance, and then click System.
You can also use the Network Identification Wizard to join a domain. To use the wizard, perform Step 1 above. On the Computer Name tab, click Network ID, and follow the instructions on your screen.
It is recommended that you use computer names that are 15 characters or fewer. If your computer has TCP/IP networking protocol installed, the computer name can be up to 63 characters long but should only contain the numbers 0-9, the letters A-Z and a-z, and hyphens. You can use other characters, but doing so might prevent other users from finding your computer on the network. If your network is using the Microsoft DNS server, you can use any characters except periods. If other networking protocols are installed without TCP/IP, the name is limited to 15 characters.
If you specify a computer name longer than 15 characters and you want longer names to be recognized by the Active Directory domain, the domain administrator must enable registration of DNS names that are 16 bytes or more.
If you rename your computer or workgroup when it is disconnected from the network, duplicate computer names might result. Check with your network administrator before renaming your computer.