Insert the install disks (that came with the computer) and restart the computer while holding down the "C" key.
If you want to do a complete format and reinstall, you'll want to select "erase and install" (or something like that). An archive and install (when "save preferences" is selected), will reinstall the OS and all programs that came with the computer without getting rid of any files or preferences on the computer (however a backup is still encouraged).