A guy reformatted his hard drive and then found out none of his Windows Media files would work. Turns out that Windows Media Player turns the “copy protection” (copy prevention) on by default when it rips CDs, so when he reformatted his hard drive the player thought he was trying to play the copy protected files on a computer other than the one they had been licensed for.
Let me say this another way: when you rip CDs on a Windows machine using Windows Media Player, it makes a unique identifier for your computer (that has privacy implications, yes, but I’m trying to make another point here).
That unique identifier is associated with a license that is stored separately from the file itself that will only let those files be played back on the one single computer that matches the unique identifier. No other devices. Ever.
(Without a lot of hassle anyway — Without having to backup and restore your licenses on the other computer — or use Microsoft’s Personal License Migration Service (PLMS) — two processes that, to date, have performed less than dependably — according to many a sad music collector….)
And it turns out there is a solution: turn it off! Change the settings on your player for now, and say “no” when it asks you about wanting copy protections (fair use copy preventions) in the future.
See the Guardian story by Jack Schofield:

When you first run Windows Media Player, it will ask if you want to keep copy protection on, and you can turn it off if you wish. If you missed that dialog box, it is still easy to turn off copy protection by going into the Tools|Options menu. Click on the Copy Music tab, and under Copy Settings, uncheck the ‘Protect Content’ box. In previous versions, this box was called the ‘Enable Per sonal Rights Management’ check box.” Turning off copy protection would seem the best idea.

Text of article in case the URL goes bad:
http://www.guardian.co.uk/Archive/Article/0,4273,4477138,00.html
Ask Jack
Send your questions and comments to Jack.Schofield@guardian.co.uk
Published letters will be edited for brevity but please include full details with your original query
Jack Schofield
Guardian
Thursday August 8, 2002
Catch WMP
I have been collecting music using Windows Media Player to copy from CDs. When I needed to reformat my hard drive, I copied all my files to CD-R, re-installed my operating system and copied them back, only to find my music would not play.
Rowan Burgess
Jack Schofield replies: Microsoft’s web site says: “By default, Windows Media Player [7.x] is configured to protect content that is copied from a CD to your computer from unauthorized use by using Personal Rights Management. When this feature is enabled, each track that is copied to your computer is a licensed file that cannot be played on any other computer unless you backup and restore your licenses on the other computer.”
Reformatting the hard drive has made your PC, in effect, a different computer. Since you did not back up and restore your licenses, there is no obvious way to play the protected files. However, Michael Aldridge, lead product manager in the Windows Digital Media Division at Microsoft in Seattle, says: “There is still a way to get these licenses back and it is pretty easy using our Personal License Migration Service (PLMS), [which] was designed to address the exact situation you outline. The customer just has to be connected to the internet, then they can automatically restore their licenses just by playing the music files in question.
Windows Media Player will recognise that the music had a license and will go out on the web and update their music files with new licenses. All this service does is note these files once had a license and provides a new one. No internet connection is required for playback after that. “If the reader is connected to the internet and this is still not working, it is most likely because they created their music collection with an earlier version of Windows Media Player (7.0) and then upgraded on top of that collection. We did anticipate this scenario and developed a tool to help them update their licenses: the Personal License Update Utility. This must be run before they upgrade their system or transfer their music files to a new PC.
If they don’t use this utility they will need to re-create (re-copy) their music CDs into their music library on their PC. Find out more information about this process at www.microsoft.com/ “You can also choose to turn off copy protection when you create your music collection, which can be done easily in any version of [WMP7.x or later].
When you first run Windows Media Player, it will ask if you want to keep copy protection on, and you can turn it off if you wish. If you missed that dialog box, it is still easy to turn off copy protection by going into the Tools|Options menu. Click on the Copy Music tab, and under Copy Settings, uncheck the ‘Protect Content’ box. In previous versions, this box was called the ‘Enable Per sonal Rights Management’ check box.” Turning off copy protection would seem the best idea.