Most laptops and many OEM (HP, Sony, etc.) desktop computers come with a factory restore image on a special partition on the hard drive. The factory restore process is normally invoked by pressing a Function key (like F11) at computer startup. There will be a message as to which Function key to press on the screen when you first start your computer. You can also refer to your computer manual, or the computer mftr.'s website, or contact its tech support to see how to restore your computer to factory condition since each computer mftr. does it differently - sometimes even between models.
These machines also usually have a utility with which to create physical recovery discs in case the hard drive needs to be replaced or the factory restore image is somehow damaged. If you did not create the physical recovery discs and/or damaged the recovery image, contact the computer mftr.'s tech support to order a recovery disc set. This is normally very inexpensive, around $20-25. MS-MVP - Elephant Boy Computers - Don't Panic!