You didn't mention the OS. If XP, you boot from the disk and do a repair installation. If done properly, it won't remove your programs or data but will replace corrupt system files and rebuild/repair the registry. Just to be sure that nothing gets lost, however, you might be well off to attach the drive to another PC and copy out your important data.
My kids were moving furniture and bumped a cord in an outlet that caused the lights in the living room to flicker and my desktop monitor to flash with a small popping noise. When the comp rebooted itself it went into a screen I've never seen before. The screen is black and states..."We apologize for the inconvenience but windows did not start successfully. A recent hardware or software change might have caused this." It followed with directions and options to restart the comp and options for 5 restart modes. The five options are: safe mode, safe mode with networking, safe mode with command prompt ( all of these options, when chosen only yielded an onscreen list of configuration specs with no options available), last known good configuration (this option yielded a failed reboot attempt that returned me to the "We apologize..." screen, and the last option, start windows normally (this option also yielded a failed reboot attempt with a return to the "We apologize..." screen. If no option is selected the onscreen countdown causes the comp to remain in a reboot loop in the start windows normally option. Any advice on how to restore my comp to working order would be most appreciated.