I assume that the PC is not part of any domain at present, and therefore on a workgroup (normally OEMWORKGROUP).
Log onto the machine as THAT machines local admin account, that was setup when 1st installed, then add it to the domain using the administrators account and password of the domain controller. Also check there is no other PC of that name anywhere in A/D or users and computers. If there is delete it or try another name for yours.
Hope this helps