If you're using Windows Media player version 9 or 10, the tool FairUse4WM will easily unlock your WMA files. It will work on Windows Media player 11 revisions before 11.0.6000.6324, so if you have Windows Media player 11, right-click on the title bar and go into Help -> About Windows Media Player to check the version you're using. Microsoft made a significant DRM improvement in revision 11.0.6000.6324, which breaks this tool. Another tool worth checking is freeme2, however, I'm unable to confirm whether it works with the newer Windows Media Plyaer revisions. Have a look at the undrm.info website for some info about these two and download links.
If you recently updated Windows Media Player (e.g. version 11.0.6000.6324 or later), the only free alternative I'm aware of is to use a sound recording program (e.g. Audacity) to record from the soundcard as you play each protected song. If you go about this method, be sure to mute all the recording devices on the sound card to prevent background noise being introduced in the recordings. Then carry out a few songs a day depending on your spare time. If you have a large collection (e.g. >500 songs), start with your favourites, as there's little point in spending time converting songs that you'll unlikely listen to on your MP3 player.