Update your Graphics Drivers.
If that doesn't work, then this Registry Hack should solve this:
1) Go to Start --> Run --> regedit and press enter
2) Click 'Edit' and click 'Find' in the Edit menu
3) Type ' DefaultSettings.XResolution ' (without quotes) and click 'Find'
4) Change 'DefaultSettings.XResolution' to 500 (500=1280 in hexadecimal).
4) Change 'DefaultSettings.YResolution' to 320 (320=800 in hexadecimal).
5) Reboot.