Windows 7 32-bit
I never install codecs on my system, the most I've done is install VobSub Filter and ffdshow. JRiver Media Center 14 trial seems to have left a lot of codecs on the system and Windows Media Player is preferring those codecs to ffdshow- even with a very high merit in ffdshow config (which seems to change back to default merit every time I open it, different topic).
Anyway I'm only sure about this if: Windows Media Player doesn't support Xvid decoding without installing a decoder, I'm pretty sure it doesn't.