If you have an older model Dell (at least a few years old) and want to reinstall Windows; press and hold CTRL+F11 when it boots up and just click ‘Restore’.
This will format your HDD and restore your computer to factory settings.
Really helpful if you lost your Reinstall CD, also you DON’T need a Product Key to activate it as it’s done for you! Happy days.
Just make sure you back up your files (music, pictures, etc) to an external HDD or flash-drive first.