For Nuvi 13xx 14xx do this.
Make sure that you have loaded Garmin Drivers on the Windows Computer.
Start Updater.exe
Make sure that the nuvi is switched off.
Plug usb cable into the computer but not into the nuvi.
Now place your finger on and hold the top left corner of the screen with the nuvi still off.
Plug in the usb cable. It should enter pre-boot mode now. (Ding Dong)
If it doesn't happen try inserting a memory card or removing a memory card and try again.
Another trick that seems to solve some issues on 13xx 14xx is holding the power switch down for longer than 30 seconds and then plugging in usb cable.