Yes, what michaaa62 works, and before you connect you n900 to your pc you might want to have the commands already written in to CMD and entered so it says 'waiting for suitable device' at this stage connect your powered down n900 while holding the 'u' key to the pc, and keep holding it until all the drivers install.

Also if you are also flashing the fiasco image then i would recommend holding down the 'u' key through the entire flash, other wise it might reboot back to maemo during half flash.

Good Luck