My cousins computer resets his Display to 800x600 each time he tries to change it. How do I fix it?

He will reset his display settings so that his resolution is bigger. Then it will tell him he must reset before he is able to apply the changes. (All fine and dandy.) Then when he restarts it will appear in his display setting for all of.. two seconds before it resets itself, without warning to a display setting that is so big that he will be unable to do anything else.My cousins computer resets his Display to 800x600 each time he tries to change it. How do I fix it?
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