If this is content you created from your own music collection, you might want to re-rip these, and encode them using something other than WMP. So that this time there is no DRM applied to the music files.
As far as I am aware, there is no program which will remove the DRM from the file. Its likely that the protection is similar to file encryption, Which simply can not be "removed" but must be decoded with the proper key.
A solution would be to re-record the music using another program.
If your sound card can do full duplex, then it may be possible for you to play the song via 'WMP' and record it via something like 'CoolEdit.'