This question was asked internally and I was told this is the answer - I've not been able to test it fully, but it appears to be heading in the right direction!!

Q. Why can't I play files that are shared by one computer but located on different computer? A. You can use Windows Media Player 11 to share media files that are located on other computers on your home network. However, additional steps are required so that these remote files will play on the devices you are sharing them with. On the remote computer, the media files must be in shared folders. For information about sharing folders, see Windows Help and Support. On the computer that contains the library you are sharing, you must grant users permission to access remote folders, modify registry key values, and monitor the shared folders located on the remote computer. The information in this section covers only granting permissions and modifying registry values. For information about monitoring folders, see What are monitored folders? There are five different procedures for granting permission to remote folders. The first procedure must be done and enables remote content sharing on the computer that contains the library you are sharing. The second procedure grants a specific user account permission to access folders on other computers. However, it will disable sharing for all user accounts except the account you specify. Also, if another user has sharing enabled on this computer, the Windows Media Player Network Sharing Service might not start if this procedure is followed. The last three procedures grant anonymous logon permission to access folders on other computers. This means that any user can access the folders; it does not distinguish between user accounts. The third procedure is for users of Windows XP Service Pack 2 (Professional and Media Center editions only); the fourth procedure is for users of all other versions of Windows XP; and the fifth procedure is for users of Windows Vista. For procedures two through five, you should only perform one of these procedures.   To enable remote content sharing   To grant one user account permission to access folders on other computers   To grant anonymous permission to access folders on other computers (Windows XP Service Pack 2 - Professional and Media Center editions only)   To grant anonymous permission to access folders on other computers (all other Windows XP versions)   To grant anonymous permission to access folders on other computers (all Windows Vista versions)

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David