Hopefully HP will answer. They also have a forum where you could ask about things but I don't know how fast you'd get an answer there.
As for reinstalling Windows, you won't need anything else than the code on the license sticker on your case and a generic Windows 7 installation media, either a DVD or a USB stick, which you can download from the link I provided earlier. You can also copy or lend such a disk from anyone, that's fully legal. Using a generic media to reinstall Windows to a <brand> computer is common practice in service shops because that way you don't have to install and then uninstall all the unnecessary programs that only waste your computing power. I've done it numerous times and it's very straightforward.
If you ask HP about their whitelist policy, they'd probably say that it's for ensuring compatibility. Truth is they want people to buy new computers instead of upgrading.