OK, this is how I managed to overcome this issue (in order to play MKV files on WDTV Live box, as we all know streaming of MKV from WHS 2011 is not working):
1. start "Computer Browser" service and set it to "Automatic" start (this part is crucial, it seems that WDTV box uses this service for auto discovery)
2. enable "Guest" acount (per user authentication is working but "No Media in folder" message appears every time when you browse shares
Does anyone have other suggestion?
BR,
MiskoR