Trouble with music files and WMP?

I recently bought a new laptop and Im trying to put all my music onto it. It worked at first for a little while, but now whenever I try to play it, it changes into an MPEG 3 LAYER file and WMP wont play it. So I delete the files and transfer them again from my old desktop as an MP3 file, and as soon as I try to play them in WMP on the laptop, they turn back into an MPEG 3 LAYER file. What is that file, and why is my laptop doing this? How can I stop it so I can play all my music?