m4p to mp3?

It can't be done. M4P files are encrypted AAC files from the iTunes Music Store. They are designed to not allow you to convert them to other formats and not allow you to share them with more than 3 computers. Apple's FairPlay encryption has been cracked in the form of something called PlayFair. However, PlayFair to my knowledge is only distributed in source code and is now very hard to find as Apple has been going after anybody that hosts PlayFair's source code. Apple forced PlayFair off of Sourceforge and most recently off of the India-based server that PlayFair moved to. PlayFair is also illegal in the United States as says the Digital Millennium Copyright Act: any cracking of encryption or copy-protection methods is illegal.

Correction: It can be done. Just burn the M4P file to an audio CD and rerip the song into MP3 format.

I've never used the M4P format on an iPod so I'm not sure. I know that you have to authorize a computer using your iTunes account to play the M4P file just on your computer so perhaps you need to do the same for your iPod.

True, you could use PlayFair but just how available is the application in a useable format? For the average user it would have to be available as binary executable files with an easy to use GUI interface. I have yet to see anything but source code for it. So for the average user, burning the encrypted AAC file to a CDRW as an audio CD and then reripping it would be the easiest solution.

Is there a windows client?