Microsoft does not supply Windows 7 ISO downloads for OEM preinstalled operating system Product Keys for Dell, HP, etc. machines
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You can borrow the exact same edition of Windows 7 Retail DVD and use your Product Key on the COA sticker to reinstall the operating system.
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Or you can use this for preinstalled OEM or Retail Licenses. This is on a first come, first served monthly basis.
Download the correct ISO file and make your own disk and use the Product Key that is on the COA Sticker.
Download: Windows ISO Downloader.exe
And here are instructions on how to burn ISO images to disk:
http://www.snapfiles.com/get/ISORecorder.html
You change the Boot Order in the BIOS to be able to boot from your installation media:
How to Boot Your Computer From a Disc or USB Drive
Then do a clean install:
http://www.sevenforums.com/tutorials/1649-clean-install-windows-7-a.html
The COA sticker is either on the underside of the Laptop or in the Battery Compartment.
With a Desktop it is either on the side, rear or top of the Computer Case.

Using the downloaded ISO file Method may require phone Activation.
"How to Activate Windows 7 by Phone"
http://www.sevenforums.com/tutorials/18715-activate-windows-7-phone.html
The above method also will require you to go to your Computer Manufacturer's website > Support and Downloads Section > locate your Model Number and Operating System > then download and install the latest Drivers for it.
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Using the USB method in conjunction with the above Windows 7 download:
http://www.sevenforums.com/tutorials/2432-usb-windows-7-installation-key-drive-create.html
"How to Get and Use the Windows 7 USB/DVD Download Tool"
http://www.sevenforums.com/tutorials/31541-windows-7-usb-dvd-download-tool.html
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And this is very good and easy to use:
Rufus - Create bootable USB drives the easy way
Cheers.