Long story short, I had to fix my friend's computer that was blue-screening after login, usually even in Safe Mode. She got it a while back and it's an HP that came pre-installed with Windows 7 Home Premium. Since she needed it to be working ASAP, my only solution after testing all the hardware I could was to reinstall Windows 7 and restore her old documents from the Windows.old folder it was all moved to. The problem is, the reinstall was done with my installation disc, which was Windows 7 Professional. During the install I tried entering the legitimate Product Key that's on the side of her computer case, but it gave me an error about an invalid SKU, I don't remember the exact wording.
I want to be clear that I am not in any way asking for an activation work-around, software crack, etc. I'm asking, is there a way to register and activate her Windows 7, somehow downgrading it to Home Premium, or will I have to locate an installation disc of Home Premium and reinstall yet again with that?