Re-installation metter

> Ciao, My name is Walter, typing from Rome (IT) and i would like to thnx
> all
> people working here to help us!!
> This is my question: I buyed a Vista Home Premium (upgrade version) to
> take
> over my xp home and became a vista user :-)!!
> First installation was ok, but now i have changed some hardware in my pc
> (motherboard an video card), thinking that i can use vista inst. disk to
> upgrade also the OS, but it can't do it, and i must reinstall the OS from
> start.
> Metter is that i have just an upgrade product key, so i can't reinstall
> vista from begin!!
> What can i do?? can i obtain a "PK" to do it or i must buy another
> licence??
> THNX for answers!!