Captain Chaos here -
Forgive the question, but have you set up libraries
within WMC on the HTPC? They are seperate libraries from the libraries you see in explorer.
That is the only reason I can think of why you cant watch a movie stored on another PC in WMC.
WMC itself has to monitor the folder on the other PC(s). I don't believe WMC libraries would "auto populate" themselves from the homegroup being set up. Please correct me if I'm wrong here...
Since everything is working properly from within the Homegroup (with WMP at least), it seems that WMC on the HTPC hasn't been "told" to monitor the folders on the Main PC (GDIFE...

). That is kinda what I was driving at in your other thread on this issue. Nevermind the network part of what I said in that other post so much as if you can play a movie stored on your HTPC on your Main PC's Media Center... If you can't then it is definately the WMC libraries set up.
Let me know if this helps