use codec packs,
first remove all codecs u recently installed on ur machine
this includes ur windows media player and all files belonging to it, this does not mean to delete them manually!
after that dowload the file below and install it:
http://www.free-codecs.com/download/K_Lite..._Codec_Pack.htm
user windows classic player to play ur files
this should solve ur problem on the other hand if this pre-exsist just play around with the options in the classic player installed with that codec pack. I used to have such problems belive it this is the most gaurnteed way to solve this issue!