black border problem in windows 7?????????

i installed Windows 7 Ultimate 64 bit on my ASUS P5KPL-CM with 4 gb of ram,Intel Core 2Duo 2.66 GHz & pci-e NVIDIA 8400 GS with latest NVIDIA drivers 190.62 but when i set my resolution below 1280x1024 black border appears on my screen and it never goes in full screen until i set my resolusion at 1440x900 the same happens with my games i cant understand whats happening this never happened in my Windows Vista Ultimate x64...................i have LCD monitor samsung sync master 732nw but now i have set the refresh rate at 75 hz so it comes in fullscreen but my games they are still running with same black border problem my mouse doesnt go past the black border many people told me to use nvidia flat scaling option but there is no such optionin my control panel..........................

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