If your music is on a separate partition then no, although it would be a good idea to do so.
If its on the windows partition then I think you have to as Vista will need to wipe the drive. Although I guess you could boot to a linux cd and just delete all the windows folders and files manually, leaving the music folder, then install Vista.
There is no way to "roll back" to Vista. You'll have to backup your user profile files and any other files you want and then clean install Vista and restore your profiles and reinstall all programs.
That's why it's never good to use the RC as your primary OS. I always kept Vista as my primary OS and would boot into RC to test things out. That way, I would not have to try and "roll back" everything.