A very good friend of mine works at an electronics store, and he had a customer cancel an order on a machine they had built, So he has given me a fresh un used genuine activation key for 7. Im running eval copy 7068.
Im sure you can buy the product key online but I have no Idea how you update/activate/upgrade.
You can't use that key with the RC version (well, perhaps you can but it would be stupid to do so). The RC version is in the process of expiring, and a valid key will not keep it from shutting down every hour and then in a couple of months refuse to work at all.
What you need to do is to get an official release version, do a clean install of it, and use your key with it. Your friend should be able to supply you with a copy (the disc isn't the big issue, the value is in the key).
What's the difference between doing a wipe and clean install to XP and doing a wipe and clean install of Win 7? I mean - Just a matter of what media you boot to, right? So why not get/borrow a real copy of 7 instead of going 'back'? Either way, you have a key.