Before start the installation, just change the two keys in Registry. Its like you are faking the Windows 7 installation that you are using the same version which you are going to install. In my case, I had to change the key to ‘ Windows 7 Professional ’. Make sure you are typing the exact and correct OS version which is written on your installation DVD. (via [Guide] How to Downgrade Windows 7 Ultimate to Professional)
Make sure you have enough of space in ‘System Partition’ (usually C drive). Allocate enough of time since this will copy your personal data first, downgrade the OS and then copy back your data.
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