Are you on a 64bit version of Windows? Do you use any codec packs? If you are on x64 then you will have a x64 Media Centre while you will likely be using the x86 Media Player. Differences in the codecs used by the different architecture players can cause different picture quality.
I would suggest the same as above, and add that no, unfortunately you cannot force an x86 Media Centre in x64 Windows, Microsoft only provide the one architecture of Media Centre with each version of Windows, though they do provide both an x86 and x64 Media Player; the x86 is just the default.