What OS are you running?
Did you already create a full image backup of your PC already and transferred it to a disc?
If it is not on a disc yet, you can not just use the image file to back up your PC. You should be able to burn this image to a DVD by using ImgBurn.
Or, if your computer did not ship with recovery discs, it should have a program by Toshiba to create a set of recovery discs to use to format your computer to it's factory condition with all original programs installed. If there is no program it is probably on a hidden partition on your hard drive and needs to be accessed on a reboot at the boot screen.
Please give more information if you need more help.