my graphics card has 848x480, which is nearer the 16:9 ratio of your screen, than 800x600.
perhaps you have this setting to? although i do have a different graphics card.
most resolutions work out as 4:3, divide 4 by 3 and you get 1.333, divide 800 by 600 you get 1.333,
you need a resolution that matches 16:9, when divided is 1.777, and 848 divided by 480 is 1.766.
perhaps come back with the list of different resolutions displayed by your card.