downgrade windows 7

Choose Run and write regedit.

Go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion

Change the keys EditionID and ProductName.

EditionID can be found in your Windows 7 installation CD under sources\ei.cfg.

ProductName will be shown when you try to install and are denied:

Windows 7 Ultimate cannot be upgraded to Windows 7 Home Premium

Here the Windows 7 Home Premium is the ProductName.

Make sure that it matches exactly.

This procedure worked like a charm for me!