Login as Administrator and perform the following:
right-click on My Computer, and select Properties
click on the Network Identification tab, and click the Properties button
set the radio button at the bottom of the window to Domain, and enter the name of your domain (which is the same as the workgroup name you entered in the server-manager). Then select OK.
you will see a window asking for a username and password with permission to add the computer to the domain. Enter 'admin' for the username (do not use "root" or any other username), and use your admin password.
After a minute, you should be rewarded with "Welcome to the domain".
Troubleshooting - Common Errors
"The Credentials supplied conflict with an existing set of credentials"
This error is almost always caused by already having an existing connection to the server while trying to join the domain. To check on this, bring up a command prompt on your Windows box and type net use. If it lists any connections to the server, do net use \servername /delete and try joining the domain again.
"The account supplied is a computer account"
This error may arise if the domain that you're trying to join has the same name as your workgroup. Change the name of the workgroup on your Windows machine, reboot, and then try joining the domain.
you may also need to change Windows 2000/XP Registry Settings
It is suggested that you check the following registry entries which should be set to (0). This is the default under W2K (but check to confirm) however under XP the default is (1) and definitely needs changing:
[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\Netlogon\Parameters]
"requirestrongkey"=dword:00000000
"requiresignorseal"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Services\Netlogon\Parameters]
"requirestrongkey"=dword:00000000
"requiresignorseal"=dword:00000000
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Netlogon\Parameters]
"requirestrongkey"=dword:00000000
"requiresignorseal"=dword:00000000