stuck in OS limbo

so i was givin an HP DV7 laptop free. they said if i can fix it then its mine. it currently has windows 7 installed but its not genuine. i asked the guy whats up with it and he said they sent him an upgrade in the mail, upgraded it but lost the product key. so i want to reformat it back to windows vista because i have the product key for it underneath the laptop. only thing is, is that the boot up thing isnt stored on the computer, so i need to find a way to work around this..

You can order a set of Recovery disks from the manufacturer - around $25-50.
Or you can install using a retail Vista Home Premium disk, and the COA Key on the sticker that should be on the machine.

You could also (assuming that the machine has a readable COA sticker) purchase an Upgrade for Win7, and use that for a clean install (cost depends on edition of WIn7 you purchase)

Your cheapest solution would be Linux - maybe Mint. Works great.