Read all of this reply for repair and/or reinstall:

https://www.sevenforums.com/tutorials/3413-repair-install.html

Microsoft does not supply Windows 7 ISO downloads for OEM preinstalled operating system Product Keys for Dell, HP, etc. machines

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As it came preinstalled:

Here are the Key/s to press at Boot to reinstall the operating system (new condition).

Dell – Press F8 which will bring up Advanced Boot Options

How to restore a Windows 7 Factory Image on your Dell PC | Dell US

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And if you do have a Manufacturer's Recovery Partition on your Hard Drive and you can Boot into Windows, you should be able to make and have your own Recovery Disks from it to reinstall the Operating System in case of Hard Drive failure.

Look under Programs > your Computer Manufacturer's name > then their System Tools or Recovery Software Headings for them, depending how it is worded.

If you can not find any reference to it, contact the Manufacturer for advice on how to make those Recovery Disks.

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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/repair 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.

https://www.heidoc.net/joomla/technology-science/microsoft/67-microsoft-windows-and-office-iso-download-tool

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