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!