Unistalling Vista - XP Disc Set Up help

Hi,

Here is my situation. My friend downloaded a non-genuine copy of Vista on my
computer. Since then the registration has expired as i didn't have a product
key, hence basically locking me out of my computer. I can use some files and
the internet with restricted and limited use.
Since then i have emailed and read that you can delete vista by
reinstalling your original program and for me that is XP professional.

I have since then tried finding my XP disc but i have conveniently misplaced
it. I have the product key on the bottom of the computer but no disc.

Does anyone know if i would be able to download the trial version for XP
professional from the microsoft website and burn onto a CD like it says. But
instead of using the product key they supply, would i be able to put in my
product key and have this as a real copy?
Will this affect my computer? will it work? will it uninstall vista?

Help!!! if anyone has any information it would be great.

Erin