I haven't seen these instructions here on how to burn a MP3/WMA disc for your Prius yet (using Windows Vista) so thought I'd do so. You may have to make sure iTunes isn't running. I think it grabs the disc insertion event and prevents the system from giving you the option to make a data disc. Insert blank disk. A dialog will come up with these options: "Burn an audio CD", "Burn files to disc", and "Create a CD" (if iTunes is installed) - select "Burn files to disc". A dialog will come up titled "Prepare this blank disc" allowing you to specify a disc title. Expand "Show formatting options" (down arrow). Select "Mastered". (This is the important step). Type in a disc title. Press the "Next" button. Drag your music files to the CD disc drive just as if it were any other drive to copy the files to it. In Explorer right-click on the CD/DVD drive and select "Burn to disc". Give it a disc title (again). Press the "Next" button. When burning is finished it will eject the disc and the wizard will ask you if you want to burn these files to another disc. Select "Finish" That's it. Now you can squeeze a good dozen or so albums onto a single CD for your next long roadtrip.
Also, it helps if you create separate folders on the CD for each of your albums, making it easier to jump to the next album via the FOLDER button on the car's CD player.