FairUse4WM doesn't work on Windows Vista so you don't need to roll back your WMP. Besides I suppose you can't roll-back WMP on Vista 'coz it comes with WMP 11 final.

You may install Windows XP onto virtual machine (either VMWare or Microsoft Virtual PC). Then re-download media files from an online store onto your Windows XP. Remeber that WMP on your Windows XP shouldn't have version higher than 11.0.5705.5043. And never upgrade your WMP!

On your Windows XP remove DRM protection with FairUse4WM and transfer DRM free files to your Windows Vista.

You may also use other DRM removal software. Soundtaxi and Tunebite work on Windows Vista x32 (haven't seen any DRM removal software that supports x64).