Windows 7 comes with example audio files. Have you tried playing one of those with Media Player?
Do you know if you allowed iTunes to convert any of your files?
What program starts if you double click an .mp3 file?
If you open Media Player help then About Media Player then Technical information, you should see the audio codecs listed in the attachment. This is for an x64 install.
You might see if opening an Administrative command prompt and type sfc /scannow would help and possibly replace a corrupt system file.
If you go to the control panel and Programs, then click on turn on or off Windows features, the Media Player can be unchecked, the rechecked later. Maybe this would reload the player.