Set Vista as default OS after XP

Hello,

I had Vista and XP on my laptop with Vista as the default OS. Then (only God knows why) In the Vista system folder I changed XP to be the default OS. When I started my laptop I used to get the option to select what OS to launch, now I see XP as the only choice. How do I fix this? Vista is still on my computer. How do i revert to what I had before?

My computer skill is about medium.

Thanks in advance