are you using one of the flashing utilities that will install the newest OS for the device?
you must charge the device completely, then unplug it.
start the flashing software on the PC, and connect the mini usb cable to the nokia n800.
while holding the home button (looks like a house), press the power button.
a usb symbol will appear on the screen (might be in the upper right-hand corner) and the flashing app will begin its work.

HTH