There are a number of options for converting non-protected files files from one format to another. It is called transcoding in techie jargon. Google for transcoding mp3 aac.
Some of what you find may also be able to strip the protection included in many iTunes purchased music files. This software may be illegal in certain jurisdictions. Use at your own risk.
The Apple-authorized method to convert an AAC protected file to unprotected MP3 is as follows:
1. Burn an audio CD with the protected AAC music (an audio disc which would play on a B&O CD player such as a BS9000 or BS3200).
2. Rip the music back as an MP3 file.
This method, like all transcoding methods, implies a degradation in the quality of music. Some people are better able at detecting the quality loss than others.
Cheers,