To Reinstall Windows
If you choose to reinstall Windows on a Dell computer follow these steps:
First, backup all your important data to an external by either selecting the folders you need to backup or using a program such as AOMEI Backupper to create a system image.
Shut down the computer. If you are reinstalling on the same HDD you will boot off the Dell Windows CD. If you are using a different HDD (say your primary failed), use the Dell CD or a flash drive with a Windows 7 image to load onto the new computer.
Turn the computer on and strike
F12and choose what you want to boot to (either USB Storage Device or CDROM drive).
Go through the steps to reinstall the OS.
Push over your backed up data if you backed up. If it’s imaged to a point you are comfortable with, then you are done now.
For Windows 10:
Use keyboard shortcut Win+I or go to Settings.
Open up Updates and Security => Recovery => Select the option called “Refresh this PC”.