Your computer should have come with backup discs for this purpose. If not, it should have prompted you to burn some. If you insert the first of these while the computer is running, it may have a “repair” option to reinstall any drivers that have been corrupted etc. To do a full restore to factory condition, start the computer with this disc in the drive, and follow the prompts. It is really quite easy, you just need to be sure that you have created backups of any files, lists, programs and settings that you wish to keep because a reformat completely erases everything on the main hard drive as it reinstalls.