I just want to confirm that performing
the two steps installation of Windows 7 works:
1. Buy & Download Windows 7
2.
Make ISO and burn on DVD
3. Perform clean installation
4. Do not input Product Key (as it is reported to be invalid) but carry on to complete the installation.
5. Restart and logon to Windows 7
6. (Re)Run installation of Windows 7 (from DVD in my case)
7. Select to upgrade (did not select to download updates from Internet).
Summary: install Windows 7 then upgrade Windows 7 to Windows 7 works :-)
By the way, I did the installation of Windows 7 as guest OS in VirtualBox VM running on Linux as host system.