Bit of a beggar, but I guess your hardware is fairly recent.
Have you any details of your graphics card e.g. make, and whether memory is 'on board', or does it take system memory. If it takes system memory, you can adjust that in your CMOS setup - it may want increasing to the card maximum. You've probably got at least 64 MB RAM, and a card using 8 MB will leave you with 55-56 MB. Confirm.
Have you always had this problem with the machine?
Also, try checking screen resolution and position.... then start looking at updating graphics drivers.
As I say, not easy without looking at it, but hope this gives you some ideas.
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PistonBroke