Answer:
NOTE: This guide may not work if you try to install Windows XP or older after you reformat.
Assuming you will be reformatting a computer with Microsoft Windows on it, here is a simple set of instructions.
First, back up any important files you want to keep, such as media files (pictures, music etc.), documents and whatever you want to keep. DO NOT back up applications (files ending in .exe .dll .sys or other) as these could be infected by a virus. You can either back your stuff up onto a USB stick/external hard drive (make sure you buy one that has enough space to hold all your files) or if you have a computer with more than one hard drive (eg. Hard Drive A has Windows installed onto it, Hard Drive B is empty), back it up on the unused hard-drive. (in the example, you would back up onto hard drive B)
NOTE: The time to complete the following will vary greatly depending on your computer, anywhere in between half an hour to sometimes over 12 hours.
Now insert your Windows OEM/boot disk into your CD or DVD drive. Click on the Restart button. The screen will go black and the following text will appear:
Press any key to boot from CD/DVD
Press any key as said above and Windows will begin to load the files on the disk.
(If you are ever prompted a second time with this during the installation, don't press anything and wait for it to disappear. Failing to do this will restart the installation process all over again).
Continue selecting your preferences until you come to a menu that allows you to chose between "Upgrade" and "Custom". If you want to reformat your computer, select "Custom" (NOTE: You will not be given that option of you are installing Windows XP or earlier).
Next you will proceed to a menu that asks you where you want to install Windows. Select the hard drive with your old version of Windows currently installed on it. If you don't know which one that is, it is usually the primary hard drive or the one with the most data on it. DO NOT reformat the drive that you backed up your files on (if you chose that option). With that drive highlighted, click on "Drive Properties" and then click "Format". Wait a bit and the drive will be reformatted. Now install Windows on the drive you just reformatted.
The rest is easy, just set up your preferred settings and create a user account.
After you are done all this, you will be logged on and your newly reformatted computer is ready for use. Just remember to take the boot disk out when your done or your computer may try to launch the install process again.