I was wondering something. This is the situation: I currently have an OEM computer that came with Windows XP 32-bit. I just called Microsoft and asked the rep: if I were to install a windows Vista or 7 upgrade onto this computer, can I later clear the system completely, and re-install the original version of windows XP from the boot dvd-rom? I was encouraged to go with the upgrade from my current XP install, not buy a new full-version OS. People on this site and a few others have recommended against the upgrades, and to do the clean installs. I have XP Home premium. If I put a vista upgrade on here, it would be to home premium also. But, the rep told me that once I install that Vista upgrade, I can never downgrade. Unless it is to the xp professional or another version (the two more expensive and higher end versions). I believe that the rep also said this applies to windows vista, if I were to try and downgrade from Windows 7. He mentioned something about XP and Vista are no longer supported. But for some issues and for a fee in certain situations there is technical support supplied. What I do not understand is: If I were to completely erase my system, erase the OS, and start from absolutly nothing and re-install windows - How can I not re-install that old version of xp from the (or a) boot cd? Would some sort of microsoft installed program prevent me from doing that? Or would this be noted around the product key, and when I tried to validate it, it won't validate? People build computers all the time as a hobby and erase stuff and switch OS's. They don't always upgrade an OS. There are people who build computers used, and do thsi regularly. Maybe since I have an OEM, it's different. But I did pay for the OEM Operating System. I did pay for the computer. And it would be reinstalled on the exact same computer. I am curious mainly due to a potentialy need to re-install a stable back-up. And what if I really don't like Vista? I want to try it more. It looks cool. But waht if after two weeks of it being on my computer, I want XP back? The answer to my question is not to buy windows 7 and install. Please humor me about Vista. This is also a concern I have about just installing Windows Vista or 7 64-bit to see what happens on my computer. I think I was told on the phone I cannot revert back to the XP 32-bit? One day I plan to sell or give this computer away. Obviously I'm not keeping anything from this computer, except some hardware. If I can't put XP back on here, and maybe put a Vista 32-bit upgrade how can this computer ever work again? What if I tried Vista or 7 64-bit and removed it to put it onto my new computer at that future date? I cannot put the old OS back onto this computer to give away or sell? Also: If I wanted to try Vista or 7 64-bit on my computer: Can I remove it and place it onto another computer? Would the validation process work? It would only be on one computer each time, I am not pirating an OS. It would probably be a home premium version. I don't really need the other ones unless they are on a cheap sale. I can/will call microsoft about this last question. I just ran out of time, they closed. Thank you. I have tried to be very clear, I hope it helps.