If it's a Dell. You might have a "PC Restore Partiton". The easiest way to find out is boot up the computer and after the "Dell" Bios screen you should see a black screen with a blue bar at the top that says
www.dell.com. If you see that reboot the computer again and at the Dell Bios screen slowly press CTRL+F11 that's the shortcut to launch the "PC Restore" utility, after that it's just a few clicks and it erases all your stuff and puts it back to the way it was when it shipped to you. If that's not an option and you don't have a Windows disk you'll have to download a DOS Boot Disk. It goes on a floppy disk and your computer will boot to it, from there you can do a Format C:.