http://panerarichang.blogspot.ch/2012/01/tip-cname-windows-server-2008-r2-x.html
I found this post very useful, after many many not useful ones which, first of all do not explain the reason and just blindly propose solutions.
Some might find my own experience useful. I found the registry key already set, but the problem was always there. XP clients cannot open the share. I removed it and put it back. Then the issue is removed.
The above post explains that that is the solution, with the reason very clearly explained. Otherwise, I would not have tried the remove/add-again solution.
