Does Prohibitiion Applly to ... ?

Mr. Moderator, Does the license change affect PCs other than home-built? Please advise your understanding on this situation:

1. Given a malware-infected, brand-name, OEM Vista Home Premium PC/laptop needing a wipe-and-reload of its OS etc., can a backup DVD purchased with, or one burned on, another OEM Vista Home Premium laptop be legally used to restore the OS of the PC/laptop, now? Am talking specifically about moving TO a Dell Inspiron laptop that was purchased with Vista Home Premium (x86) in 2007 and upgraded with SP 1 and SP 2 plus patches since SP 2, etc., and has its original license sticker still attached (Product Key for Vista HP) , but where the OEM Vista HP backup DVD that was created after the purchase has been lost or stolen in a cross-country, whole-household move; and now, the Dell needs installation of (probably a wipe-and-re-install of) Vista HP (might this be done by use of somebody else's, bought from mfr. or burned from OEM, Vista HP DVD, now?), based on registering the change with the target Dell laptop's OEM Product Key. Is this a. workable; b. legal or probably not illegal?
2. And, a further complication, a helpful tech guy some months ago, "cured" a disabling malware incursion by a wipe-and-reload on this target Dell, with Vista Home Basic SP1 (x86). and its Home Basic Product Key (of unknown-to-the-Owner legality - although Windows Genuine Advantage has raised no objections to installing updates, including Service Pack 2). My objective is to reverse the downgrade from Home Premium given that I have the original OEM license and Product Key for Home Premium, and then to upgrade with mnewly purchased Windows 7 Professional upgrade disc (still in the shrinkwrap). or maybe I will wait for Windows 8. Or, maybe I ought to (can one?) upgrade to 7 from Home Basic? And, can a DVD burned on a Toshiba (or anything?) OEM PC/laptop be used to perform this upgrade on this or any Dell?
Wow- the technical problems are bad enough, and when you cross-multiply with the licensing problems, it gets complicated. shocked

Your best-educated opinions are respectfully requested, and will be much appreciated (and obviously, there is no intention to represent such opinion as professional legal advice).