Discribe conversion problem: mp3

I'm running 5.0.27 on Mac 8.6.
Some mp3 files refuse to be converted into AIFF format. When dragged
to the interface, the pull-down menu selection switches from Audio
CD, which I've selected, to "Build a Mac HFS."
The mp3 files in question were created on a Windows machine. My
question is this: Can I do anything to convert these files so that the
software will process them like most mp3s and convert them into AIFFs?
Thanks,
Paul