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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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Use Heidoc:
Download the correct ISO file and make your own disk and use the Product Key that is on the COA Sticker. This is on a first come, first served weekly basis.
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
How to access Boot Menu:
https://neosmart.net/wiki/boot-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.
It may require that you use 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.