I have done a clean install on three systems for customers using "Upgrade" Windows 7 discs. Like the one person told you, don't put in the key when asked during install and don't put in a password. It will finish installing and then you'll be at the Windows 7 desktop. Let it do updates it asked, until there are no more updates to install. Restart the computer and then go to the Start Orb, Computer, Right-click on Computer, go to Properties. You'll see the Windows Activation section below when you use the slider bar on the right. Click on Activate Windows and enter your key. Click Activate Now and you should be all good.
If not go here to see a couple of tricks this guy suggests at his website, Paul Thurrott's website. MS is aware of these methods and approve them.
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These do work! Good Luck!