1680x1050 is not a common resolution. You probably have a 23" widescreen monitor and it's probably a Viewsonic?
To get Windows to know this is the resolution you need to run on this monitor, go to the monitor manufacturer website and download the drivers for the monitor.
One thing monitor drivers do is tell the OS what resolutions the monitor supports, and once you've loaded the drivers you should be able to make this selection without issue.
Also, the connector you've plugged the monitor into can make a difference. For instance, using the Viewsonic VA2232wm monitor on a VGA connector, it'll usually detect properly in Windows XP and give the correct resolution options. However, using a Display Port to VGA adapter, and while it gives a dizzying array of possible resolutions, 1680x1050 is not one of the options until I load the monitor drivers.
UPDATE clear instructions for updating monitor drivers:
- Begin by downloading the monitor drivers from the manufacturer website. There may be updates or drivers that are written specifically for Windows 7. Extract the files to a directory. What you want is an .inf file that contains the actual drive information.
- Open the Control Panel. Go to Display, and then Screen Resolution.
- Right-click on the monitor showing there and select Properties.
- Click on the Monitor tab and click the Properties button.
- Click on the Drive tab and then the Update Driver button.
- Select "Browse my computer for driver software" and then browse to the location of that .inf monitor driver file and click the Next button.
If the driver package contained a more than one inf file, you'll probably see several different monitors listed in the following windows. Select the correct one and choose to install it.