A little more info...
1.) whats your screen resolution?
sometimes if you increase or decrease the res. you can get rid of anomilies.
2.) what's your color depth?
i.e. 256, 16bit, 32 bit, try varying those as well ESP. if it's set to 256, but it's a balancing act between depth and overall performance.
3.) (prolly should be #1) do you have the latest driver for your graphics card?
if you got the card new a few months ago (or recently, if its an older design) the manufacturer has probably created updated drivers.
OR....
/me puts on flame retardent shorts
GET A REAL COMPUTER
as always with free advise, your mileage may very.