Nikon CoolPix 800 Product Support Bulletin page 5

When the computer is unable to communicate with the camera, the
upgrade will stop and start repeatedly. In extreme cases, this process of
interruption and resumption may repeat indefinitely. Should this occur,
press the "Ctrl," "Alt" and "Delete" keys on the computer simultaneously
to force the program to quit. Restart your computer.
If this problem occurs, your computer cannot be used to upgrade camera
firmware. To upgrade the firmware, you will need to take your camera to
the nearest Nikon-authorized service center.