Hi I fine this on another site:
"here is a “simple” hack which allows you to embed the downgrade procedure in Windows 7.
In the Search Box, type regedit and press Enter.
Navigate to this registry key:
HKEY_LOCAL_MACHINE\Software\ Microsoft\ Windows NT\CurrentVersion
In the right pane, click EditionID and ProductName and give them the value which corresponds to a lower Windows 7 edition. Here is an example:
Windows 7 Home Premium (downgrade from Windows 7 Ultimate to Windows 7 Home Premium)
ProductName: Windows 7 HomePremium
Start your Windows 7 installation by inserting your Windows 7 Disc.
Now, Select Upgrade as type of installation. Follow the instruction on the screen.
Reactivate your operating system. "
Text "HomeEdition" and "HomePremium" must be without spaces!!!