do I need Vista disc?

I'm quite new to this, and I'm very sorry if this is a stupid question but:
I've just bought a laptop with vista on it and I installed XP as a secondary OS. Now, to complete the the installation and to make a dual boot, I need the vista installation disc. I'm not sure, but on my X: partition I probably have vista, but i can't boot using that partition. With my laptop I've got vista home edition but not the cd. Is there a way (if I have the vista installation on my HDD to boot the vista from there, or I'll have to buy a cd?

-or how can I boot from (if there is one) the copy of vista from my HDD?