I first tried using the WMP to stream movies into my PS3 and it was nothing but hassle after hassle, with it either burping, farting or indexing it never worked efficiently and became frustrating very quickly.
There are other media servers out there that should work with the xbox just like the ps3.
I just googled "XBox Media Server" and came up with options that should work much better than that WMP or WMC.
I don't use the WMP at all but use the PS3 Media Server to stream into my PS3 and then into the TV and it works very well.
Here is a couple of examples of the google search
How to Stream Media to an Xbox using PS3 Media Server Plex - A Complete Media Solution
You can research further but IMO you should dump the WMC and WMP and use some other more reliable media server and save yourself all the hassles.