Click on the Install now button in the center of the screen, under the Windows 7 logo.
This will officially begin the Windows 7 clean install process.
Note: Do not click the Repair your computer link at the bottom of the window even if you're completing this clean install of Windows 7 as part of some larger repair project for your computer.
Important: If you're performing a clean install of Windows 7 as a solution to a major problem but have not yet tried a Startup Repair, do that first. It could save you the trouble of completing this clean install process.