I've got a laptop (hp pavilion) that had Windows Vista on it (sadly). I'm replacing the hard drive since the old one is shot. On the bottom of my computer, there's information about the original vista that was installed on it. Is there a way for me to download vista and then use that number to validate it or something?
Yes, you can use an OEM key and reinstall the original operating system. You can find more information at this previous post:
Just make sure you stick to the same version, ie., Windows Vista Home Premium or Ultimate, etc.