Get a new graphics card was my first thought, do you have any high graphic games such as WoW etc? If so, play them and if you see pixels showing, it means your Graphics Card has overheated to the point it has actually melted.
Source: Past Experience.
Also would be a great idea to upgrade your graphics card driver incase that is the issue. By the sounds of it you have an old computer as it's WInXP, time for an upgrade me thinks :)