Don,
In most cases it is the graphics card or graphics card driver.
Often updating the graphics card driver resolves the issue, but this is not always the case, or even possible... depending on graphics cards and computer manufacturer.
As mentioned in the previous reply to this post the system check utility is a good place to start. It will report your current graphics card. The firtst step is to update the driver, if possible.
In general NVIDEA and ATI graphics cards tend to work, driver updates usually resolve issues. Integraged Intel graphics cards have mixed results and applicable driver updates are spotty, especially in older models..
Mark
