sounds like the graphics card to me.
try reloading the drivers for the graphics card, you may need to delete the existing ones first.
if that does not work, delete the existing drivers and then physically remove the graphics card, replace it and switch on. Windows should identify new hardware and either load or ask you for the drivers. this is as near a "add new hardware" situation as you can create. If that does not work it is either your graphics card is damaged, your monitor is faulty or possibly the motherboard agp slot.
the other possibility is your memory. have you added or changed it?, remove memory and reinsert. I would try this first if reloading graphics drivers does not work.
try the monitor on a different pc or a different monitor on the same pc.