A while back I upgraded to vista, but I didn't have a activation key. So I made a back up disk and I am now going to go back to XP. I was wondering if my back up disks that I have with all my pictures and music will still be transferrable when I download XP. Thank you

How did you make the backup disk? Did you use a backup program or something that creates a disk image. It's likely that you don't have a complete backup and couldn't restore your operating system from the backup if you wanted to. There are programs out there that will create an image: Norton Ghost, Acronis True Image, others. However, even if you restore the system, you might have to reactivate Windows, depending on what has changed in your system.

I'm wondering why you don't have an activation key. Not having a legal activation key could mean that you risk not being able to update your system and that would make your system vulnerable to being used as part of a botnet. There are programs (free ones) that will retrieve the activation key that was used to load your system. Google "retrieve Windows Vista activation key" or "retrieve Windows XP activation key".