Remember that you need to legally own a valid qualifying previous version of Windows
to use a Upgrade Windows 7, and to stop using (or uninstall) the qualifying OS while you have the upgrade installed. Microsoft only made doing a clean install from a upgrade Windows 7 possible to make it more convenient so you do not have to reinstall both the old Windows version (ex: Vista) and upgrade to Windows 7 everytime you needed or wanted to reinstall.
If you do not legally own a valid qualifying previous version of Windows
, then you are violating the terms of Microsoft's Windows 7 End User License Agreement
and could get your product key number invalidated by Microsoft. Plus, it is considered illegal.
For more about this, please see: Microsoft SMB Community Blog : Regardless of what any hack says, a Windows 7 Upgrade is an Upgrade. What you need to know.