Win7 ultimate installed, downgrade to Pro?

That's good news. :)

What was it that got the workaround running?

well i did disk clean up and removed all the junk I had, then mounted the disc using power iso with "auto mount" selected and the cd started to run !

Aha, as suspected some remnants of uninstalled SP1 were interfering until cleaned up.

Glad it worked.

Sorry to revive an old thread, but I'm having exactly the same issue, (it's currently running Win7 Ultimate x64 SP1, which I don't have a key for) and the workaround isn't working for some reason. I've tried the registry edit to Home Premium (which I do have a key for) but it still doesn't seem to be tricked, it still knows it's running ultimate when I try the repair install. Could it be something to do with it being 64 bit? Does that require a different name in the registry?

There are two keys which must be changed in regedit while it is Run as Admin, and read exactly HOMEPREMIUM for the EditionID, and Windows 7 HOMEPREMIUM for the ProductName.

If there is a Product ID present I would back it up and then delete it since it is not present in any of the successful workarounds I've seen done over the years.

Deleted the Product ID as you said, and made the changes but I still get this:

Did you reboot the PC after the regedit?

No, should I have?

Possibly

I tried it after a restart, same problem.