There is no 100% guarantee that after rolling back your Windows Media Player, you will be able to use
FairUse4WM. Please also note that you will be required to delete the DRM folder and this will kill all your licenses. Make sure you can re-acquire them.
Another problem is that some content providers force you to upgrade WMP. Otherwise you won't be able to get any music, videos from them.
How to roll back WMP:
How to uninstall and roll back Windows Media Player (Windows XP)
How to roll back or uninstall Windows Media Player 11 (Windows Vista)
Please note that instead of rolling back Windows Media Player to a previous version you can install a fresh copy of Windows XP. You can install a standalone copy or use a virtual machine (either VMWare or Microsoft Virtual PC).
Install Windows XP (assuming you have your Windows XP install/upgrade disc) onto your virtual machine. Then run the beta version of WMP 11 if your purchase requires it. If not you may try to use WMP 10 and remove DRM from within the virtual machine and simply transfer the DRM-free files to your Windows XP/Vista music folder.