Before beginning the format process, back up any important data. When a drive is formatted, everything is lost.
Insert the bootable media into your computer and reboot the computer. When the computer turns back on, it will be at the MS DOS prompt.
At the MS DOs prompt, type in "format c: /s" without quotation marks. The system will then ask for confirmation to perform the format. Type in "y" without quotation marks to continue.