Hi, this sounds like Video/Graphics Card problems.
If you are ok with delving inside your PC, you could start by giving the inside a good clean out, dust/fluff etc. Then if you have a graphics card, as opposed to on-board graphics on the motherboard, you should remove and then refit the graphics card, making sure it is firmly seated in its slot - while you have it out give the slot area a good clean too.
Also check all connections inside and outside the PC, especially the external Keyboard connection - is the key board USB or PS? Can you try a different keyboard?
Just for starters, no doubt others will come to your rescue also!