I'm not sure exactly why you've hit this issue, it may have something to do with the WDDM driver which has been updated to 1.2 for windows 8.
I had a similar issue in that changing to win 8 meant that my monitor would only run at 60Hertz instead of the 75Hertz it once did under windows 7. You might an option to make some changes however. If you look under the monitors properties (via the CCC) and see a checkbox for EDID ( Extended Display Identification Data) untick the box. This will allow you to now change the resolution to one you actually want.
If the above doesn't work try checking google to see if your monitor has a firmware update.