Hard drives are the computer primary storage devices, and to use them, they need to be formatted. Formatting a disk implies configuring it via the file system so that Windows can store information on that disc.
Hard drives are usually formatted on the new computers that have Windows. However, if you want to if to extend the storage with additional disk, you might need to reformat the computer.
To format the hard disk and reinstall Windows, insert the Windows installation disc (this is commonly referred to start from the installation disc) and restart the computer. During installation, you can recreate a partition on the hard drive and reformat it, and then reinstall Windows.
The procedure will erase all your files and programs, so before you start with formatting, make sure to make a back up for your data and program files.