It seems that Windows 7 have an issue with adding user to the Domain.
It will prompt an error like below:
The trust relationship between this workstation and primary domain failed...when this happened you'll not be able to add the user from Domain to PC.
Actually there a hot fix for it issued by Microsoft..
You can read all about it here and then Request the fix, the link and password for the download will be emailed to you in a few second.
here is the link to the Hotfix readme Click ME!!
I also already added the link to my APPs TIPs pages for easy reference later.
1. Install the hotfix (it just a small download),
3. add the user as usual from the domain
Hope this helps someone out there.