Chances are then, you don't have one. You might have a restore disc, but that won't work. I don't think vendors are kind enough to supply you with a regular retail copy of the OS anymore these days. Or if they do, only at a premium.
When you get the copy of Windows 7, it'll depend on what it says on the box/disc. If it says 'Upgrade' then the only thing it might let you do is do a fresh install to a new partition (which would include a second internal hard drive, if you have one), setting up a dual-boot environment and using the existing Vista install on the hard drive as its verification. I honestly don't know, I've never personally gone through the process of upgrading Windows. I'm still on XP.
If it won't verify against the installed Vista, then you'd have to go buy a retail copy of it (although if that were the case, just buy the retail version of 7 instead - Home Premium if you don't need it for an extra-powerful* computer, Pro or Ultimate if you are using such a machine). If the copy you get actually isn't the upgrade version, then you have nothing to worry about as it won't need to verify against anything.
*'extra-powerful' in this case, refers mainly to the RAM - Home Premium is limited to 16 GB of RAM, Pro and Ultimate can have up to 192 GB (not that any consumer setups have nearly that much yet, but HP does have the option for 24 GB in one of their customization options, and that would require Pro or Ultimate to take advantage of the extra RAM). Pro and Ultimate also have the option of using Windows XP mode.
