have you tried to shut down your n900, take battery off, put it back, restart and try to connect again. If that doesnt help then i would advise to go windows device manager and from view menu select show hidden devices and delete all other but root hubs, host controllers (Be Advised! this deletes all the usb plugged devices drivers what you have connected to your computer from day 1) also disconnect usbs before that operation and also windows reboot after that is recommended. I do not know if something bad happens from this but it has helped me couple of times with usb devices that do not anymore get reconized by windows.