How can I use the complete screen for low resolutions?

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I use a Siemens-Fujitsu Lifebook with Windows XP Professional Version 2002
Service Pack 1.
The graphics controller is an Intel(R) 82852/82855 GM/GME graphics controller.
The (default) settings of the screen area is 1400 by 1050.
For some applications (e.g. connection to beamers) I change the settings in
display properties.
When I change the screen resolution to lower values the visible desktop gets
smaller.
When I change the screen resolution to 800 by 600 only a fourth of the
complete screen is used.
How can I use the complete screen for resolutions lower than 1400 by 1050?