Welcome to Computerhope, and Merry Christmas to you too, xurich.
Is there an option within the nVidia drivers (is it the Detonator drivers?) to change the video mode?
I don't recall exactly what it's called (it'll be different with nVidia anyway), but with the ATi Catalyst software you can change the display ratio, scaling and other options.
Have a look around in the drivers and see what you can find.
Hope this helps.
Calum.