Use this device as the primary monitor is greyed out

I've just bought a new Medion 19" LCD monitor to be used with my relatively new Medion MD2900 laptop, which has a GeForce4 440
Go (64Mb) video card. Now, I intended to use this external monitor as the
primary display device for the laptop permanently and then perhaps
sometimes extend the desktop over the laptop display, using the laptop as
secondary. For some reason my O/S (Windows XP Prof.) will only let me do it
the other way around, i.e. the laptop display as primary with the desktop
extended over the external monitor. XP has greyed out the 'Use this device as
the primary monitor.' option for the external monitor, forcing it to be
unchecked and has greyed it out also for the laptop LCD forcing it to be
checked. I can't find anyway to get it to allow me to change this (what's
the point in this option if you can't change it?). I have the latest
monitor driver and also have the latest NVIDIA driver for my card. Also, even though there are what look like monitor symbols on the function keys of the laptop (accessed by pressing a 'Fn' key at the same time as pressing the function keys), none of them succeed in switching between primary displays and there is no related option in the BIOS configuration...

Can someone please tell me where might find a registry entry or some other modification that will allow me to close my laptop and use my 19" LCD monitor as my primary monitor?

Thanks,