To fix this,you got to have an Operating System with you,either in the form of a Disc or an Image file of the bootable disc.To start with,get the Disc with the OS inside your Optical drive and restart your PC. Make sure that the Optical Drive is the first boot device by going into the BIOS. After that,press any key when asked to do so,and the OS will begin installation. Steps after this shouldn't be hard to understand. If you have any confusions or doubts,please do ask...