QTFairUse for ITunes will allow you to convert protected m4p format files purchased in the iTunes Music Store to unprotected m4a files, playable everywhere.

Usage:
Total conversion
From version 2.0, QTFairUse6 supports fully automated conversion of all protected tracks in your iTunes library.
For that, start QTFairUse6.exe with no commandline parameters.
The program will confirm whether you want to convert all the files in your library. Press y and Enter to do that.
Next it will ask if you want to skip already converted tracks (e.g. from the previuos run). Press n and Enter to owerwrite them, y and Enter to skip.
After that, the script will begin conversion of all files in the library. Converted files will have m4a extension and will reside next to their m4p counterparts.
To stop the conversion in process, press Ctrl-C in the console window. You can also close iTunes.

Batch conversion.
If you want to convert specific m4p files, pass their filenames as the commandline parameter. You can do that by e.g. dragging and dropping the files on QTFairUse6.exe. QTFairUse6 will convert them and exit. You can find converted files next to the originals, with m4a extension.

Known issues
iTunes 6.0.4 reportedly can crash after around 10 seconds of playback. The cause is unknown. Workaround: upgrade to version 6.0.5.
With disabled sound output iTunes may become unresponsive. This is normal; the produced tracks are still correct.
iTunes 7.0.0 sometimes produces a COM error. Fix: run "iTunes.exe /regserver" to reregister iTunes COM.
Sometimes QTFairUse6 cleans up incorrectly and iTunes crashes. Just restart iTunes and try again.