From http://www.mp3licensing.com/help/developers.html
"I have my own/third party mp3 software. Do I need a license?
Yes. Use of our patents is not related to a specific implementation of encoders and decoders, which means that a license under our patents is needed."
So even writting your own decoder from scratch requires you to have a license, never mind making use of one that might already be part of Windows (which would be a 3rd party in this case).
Just use Ogg and to hell with them.
"I have my own/third party mp3 software. Do I need a license?
Yes. Use of our patents is not related to a specific implementation of encoders and decoders, which means that a license under our patents is needed."
So even writting your own decoder from scratch requires you to have a license, never mind making use of one that might already be part of Windows (which would be a 3rd party in this case).
Just use Ogg and to hell with them.