If you would like to have a clean installation of Windows 7, just follow the instructions below:

( Suppose you want to install Windows 7 to drive C)

1. Extract all files from Windows 7 image to drive D, let's say D:\win7
2. Copy D:\win7\boot to C:, copy D:\win7\sources\boot.wim to C:\sources
3. Open CMD, type in
D:\win7\boot\bootsect /nt60 C:
4. Reboot will bring you to the installer
5. Click on next and select "Repair my computer"
6. Enter command line
7. Execute:
d:
cd win7/sources
format c: /q /fs:ntfs
setup

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