Chances are you're trying to play h264-encoded files. The simplest solution is to
get CCCP and uninstall your previous codecs. If you have something against CCCP, you could also hunt around for a good version of FFDShow that support h.264. Yet another alternative is to use VLC as your media player; it also supports h.264. At this time there aren't any other feasible ways to play h.264-encoded files.