Computer Help

I just got a new Samsung Syncmaster 943BWX monitor for Christmas, I'm trying to set it up as my second monitor but for some reason the monitor has decided that its going to have a resolution of 1280x800 instead of 1440x900. I've gone into the settings from Microsoft and it says its set at 1440x900 but in the monitors own menu it says otherwise and won't let me change it. Any help would be appreciated