Use the Video Lan Client to generate your iTunes Music Store license key, then deDRMs to strip the copy protection—but not the personal information identifying you as the owner of the music—from protected .M4P songs. Although the deDRM interface is clunky, at best (drag and dropping individual files onto the program is tedious), it works. A pre-compiled binary for Windows is linked below.
At least one worry about my impending Windows reinstall has gone away.
Download – DeDRMS.exe [SFSU.edu] [via BoingBoing]
Read – VLC Download [Videolan]