The solution:

Download the latest maemo flasher (here: http://tablets-dev.nokia.com/maemo-d...-downloads.php), and the latest firmware file (.bin) (here: http://tablets-dev.nokia.com/nokia_N900.php).

Put the firmware file (.bin) in /program files/maemo/flasher-3.5

Then open the flasher, type in: flasher-3.5 -f --flash-only=kernel -R -F <firmware-image>

The flasher will wait for the n900 to connect.

Put the micro usb in the n900, slide it open, press and hold "u" then put in the usb cable in the laptop.

It should reflash the kernel, no files or apps or anything else lost

have fun =)