From: boingboing

By default, Windows Media Player encodes your music collection using your machine’s unique key, so that you can’t share, loan or give away the tracks you rip to your machine. What that means is, if you have some file-system or OS corruption and reinstall from scratch, then restore your music collection, it will be unusable. You won’t be able to play the files. There’s a backup utility that’ll preserve your license keys, but if you fail to employ it, you’re SOL — MSFT’s position is that you need to start over from scratch at that point, re-ripping all the CDs in your collection.

Link: The Guardian