Usually you need a set of DVDs from the computer manufaturer to do a fresh install. Call technical support for your comptuer and ask for a set of disks. They may charge a small fee for shipping/handling/production, but it's cheaper than a new computer.

Follow instructions to install, and start by inserting the first "boot" DVD and powering on the computer.

Save your data onto a DVD, CD, or USB stick first, including photos, as you will lose these. Better yet, make 2 copies and make sure you have it all.

What computer do you have?

If it's a self built machine you have to reload the windows the old fashion way.
If it a pre-assembled machine like HP/ACER/COMPAQ they should have a button to press on start up to access the recovery feature. Very few companies still supply recovery disks. They give you the option to create it yourself..