How to switch back to Win XP without reformatting

I recently installed windows vista on my laptop and used it for 2-3 months.
Now after experiencing a lot of bugs in the windows vista I want to install my old XP back but the problem is that I can’t.
When I insert my XP CD and try to install it the system says you already have the latest version installed. My friend told me to format my pc and then install it but that means re installing all my required software and also taking a huge back up.
Is there any easy way to get my XP back without formatting my computer.
