I assume you mean under the monitor tab, where it says screen refresh rate? Not only is the box not checked, but it's (oh, what's the term...?) not available to be checked.
Under the adaptor tab, where it lists info on the card, there's a "List All Modes" button. My choices are:
640x480, 256 colors, 60Hz
800x600, 256 colors, 60Hz
640x480, High Color (16-bit), 60Hz
800x600, High Color (16-bit), 60Hz
640x480, True Color (32-bit), 60Hz
800x600, True Color (32-bit), 60Hz (currently using this one)
640x480, 16 Colors, Default Refresh
800x600, 16 Colors, Default Refresh
Under the nVidia tab, there is nothing useful. Mostly driver info, plus some options for full screen overlay that are blanked out.
Go to the Control Panel -> Display -> Settings (click on Advanced) -> Monitor and the box should be there.
And if you don't have the latest drivers for your video card, get them.