You may just simply have some codecs "stepping on eachother's toes" so to speak. If you have downloaded a large codec pack such as K-Lite Codec Pack, this is frequently a problem. Those codec packs are really meant to be minimalistically installed, and added to when needed.
I recommend the following media software, which will allow you to play most all of the file types that are floating around, all in one player, instead of several:
1. Media Player Classic
2. QuickTime Alternative
3. Real Alternative
4. AC3 Filter
This combination of apps. play your music files, video files, and if you please, it will auto play CD's and DVD's.
It also gives you quick options to switch the renderer for the current file type being played. It's open source, and updated often.Also, QiuckTime and Real Alternatives will prompt you to install Media Player Classic, make sure to untick the option to do so in each of the installs.
I don't recommend installing VLC player and Media Player Classic, to many conflicts, if a certain filetype gives you trouble, look at the file extension, and search for it......it's out there and you don't need to install a big codec pack to enjoy your media entertainment.
If you do have a Codec pack, I recommend uninstalling it, along with QuickTime and RealPlayer, restarting your PC, and installing the programs I listed above. NOTE: Always remember to do a system restore point before undertaking tasks such as this.
Hope this helps,