Usually , it comes with recovery drive/partition. If you have not deleted that folder, you should be able to run recovery during startup ( dont know what key it would be ) . You can find the right key when you start your computer ( it would say  sth like  "press ? for  recovery mode "  where ? would be the key) . You may be able to go  to the recovery folder and run it manually too. (double click)
Otherwise, you can see the back side(sometimes on the front ) of the CPU case and there would be a sticker of microsoft  with the registration key for your operating system. Find bootable CD/DVD for that operating system ( can borrow from fren ) and use your registration key ( the one on the back  ) and follow onscreen instructions.