It might be nessecary to adjust the xconf. Some find this easy some do
not. It isn't a fun thing to do that's for sure.

Open a terminal and type,

sudo dpkg-reconfigure xserver-xorg

follow instructions, but keep the info of you graphicscard and monitor
at hand. It will make a backup of your previous xorg.conf so if you
make a mistake you can still go back (it might be handy at this time
to install mc first through synaptic it's a norton commander clone and
it will make working from the cli a lot easier when things go wrong)

after you are finished, restart your xserver by pressing ctrl alt
backspace, or log out or restart the system.