I use VLC ,it is the only player that play video good but other players(windows media player , bsplayer , media player classic) have poor quality. Whatever codec i installed (Win7codec , K-lite) the quality is the same . I think that the built-in codecs doesn't let installed codecs to decode.
Example: if i play a .avi file the quality is the same with a codec installed or without it but if i play a file that windows media player can't open with built-in codecs like .mkv , the installed codecs begin to decode and the quality is good.