You can erase the data on your hard drive by formatting it. In Windows Explorer, simply right-click the drive and click Format. This will delete everything on the drive. To do this to your drive that contains Windows, you will need to perform a fresh install instead or remove the drive and format it from another computer.
You can also use special software to wipe out the computer hard drive securely if you need to ensure the data cannot be recovered. The tools allow you to format your drive, but it adds the ability to overwrite the data on it multiple times so it cannot be recovered.