I have some iTunes files with the file type
AAC audio (protected) on Windows 7. The problem is that these songs only work with Apple software, and I want to remove the DRM.
Apple offers a feature called iTunes Plus to upgrade most purchases to a DRM-free version for a small fee. This link will bring you directly to the appropriate section in iTunes to do this.
With iTunes Match you can re-download them in iTunes Plus quality without DRM protection. And it's just $25 a year, definitely worth if you have more than an album with DRM. http://www.apple.com/icloud/features/
Here's an article that lists three ways to do it. The first way involves burning audio to CD and re-importing, which would be lossy/transcoding. The second and third methods appear to be software-based techniques to remove the DRM from the files without transcoding. I have not tried any of these methods.
Burning to Audio CD and re-ripping is not 'lossy' - yes you change codecs - however, you aren't losing quality going up to Audio CD from your 128 bit files.
When you come back down from Audio CD quality - you could lose quality, however, you could use 'Apple Lossless' or just keep them as AIFF or WAV if you'd prefer. Then you haven't lost any quality from your original files - they are just LOTS bigger :)
This answer only applies to someone with a Mac. I don't believe in paying iTunes again for something I already bought once. So, in case it helps someone else:
I've been having success with the program Virtual CD-RW for Mac... there's a 15 day fully functional demo period. (http://www.macupdate.com/app/mac/33355/virtual-cd-rw)
Basically, it tricks your computer into thinking you've mounted a blank CD. So I just filtered out all my iTunes music whose Kind is Protected AAC (bought before they removed DRM), split it up into CD-sized playlists, and burn and import playlists one by one. Just a modification of burning all your songs to an audio CD and then importing.