I'm afraid it sounds like either your graphics card or your monitor is failing. First thing would be to see if you can find updated drivers for both (especially the card) in case it is a driver issue. If this doesn't help then can you borrow a monitor from someone? Then you can plug it in and see if it's the monitor. If the picture is still bad then it is time to buy a new graphics card. These are really easy to fit and except for high performance games cards, very cheap.
there are probably something wrong with my monitor because no shadow at the beginning of using my computer but shadows appear gradually about 10 minutes later. it's almost 3 years old...think it's getting old. well can my monitor be repaired? or shall i buy a new one?