I read one book Windows 7 secrets.And got some inside details of how to Clean Install Windows 7.

Undocumented Method for a Clean Install of  Windows 7 with Upgrade Media.

To perform a clean install of Windows 7 with Upgrade media, you need to install 
Windows 7 once using the Upgrade Setup disk, but without entering your prod-
uct key during Setup. Then, once you’ve loaded the Windows 7 desktop for the 
first time, you can run Setup again from within Windows 7 and choose Upgrade 
(even though you’ll be “upgrading” to the exact same version of Windows 7). 
Allow Setup to complete a second time, and then you’re good to go: you can 
enter your product key after the second Setup routine is completed and activate 
Windows 7 successfully.

Here are the complete instructions:
Step 1: Install Windows 7
Boot your PC with the Windows 7 Upgrade DVD. After the preliminary loading 
screen, install Windows 7 normally. When you are prompted to enter your prod-
uct key, leave the product key field blank, deselect the option titled “Automatically 
activate Windows when I’m online,” and then continue.
Setup will install Windows  “Clean Install” Once 
Windows 7 is successfully installed and you are logged on, you’ll be presented 
with your new Windows 7 desktop. Don’t get too comfortable, however, as you’re
about to do it all again. 

Step 2: Upgrade from Windows 7…to Windows 7
If you try to activate Windows now, it will fail because you’ve performed a clean 
install of Windows 7 and you have only an Upgrade product key. That means you 
have 30 days during which you can run this non-activated version of Windows 7. 
But why wait 30 days? 
According to Microsoft, Upgrade versions of Windows 7 support upgrading 
from “a compliant version of Windows, such as Windows 7, Windows Vista, or 
Windows XP.” Well, you just installed Windows 7, so why not just upgrade from 
that install? That’s right: you’re going to upgrade the non-activated clean install 
you just performed, which will provide you with a version of the OS that you 
can, in fact, activate.
To do this, just select Computer and double-click on the icon for the DVD drive 
that contains the Windows 7 Upgrade media. Run Setup again, this time from 
within Windows 7. When you get to the appropriate phase of Setup, choose the 
Upgrade option. Windows will install as before, though you might notice that it 
takes quite a bit longer this time. (Upgrade installs take up to 60 minutes, com-
pared to 20 minutes or so with clean installs, and reboot at least one additional 
When Setup is completed, enter the username and password you created during 
the first install and log on to Windows.
Now that you’ve “upgraded” Windows 7, product activation will actually work. 
To activate Windows 7 manually and immediately (unless you told it to do so 
during setup), from the Start menu, type activate in Start Menu Search, launch 
Windows Activation, and click Activate Windows online now.

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