The only way I found to do this is through the command line:
Net localgroup administrators /add DomainName\UserName
> Hi,
>
[quoted text clipped - 10 lines]
>
> PS: I managed to add the Windows Vista box to NT Domain.
terra - 30 Jan 2008 03:23 GMT
the olny way it worked for was the following steps
Add domain user as administrator to the PC
1) Start the PC with username which got admin privilage.
2) Start user accounts in control panel.
3) Give domain username and domain name in the place which is given.
4) Select Admin privilage from the 3rd option for the user rights.
all the other methods, did not work.

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terra