Green Monitor Screen

A while ago my friend was on his computer when the monitor screen went green. Thinking it was the monitor he left it and has been looking for a new monitor. Yesterday he decided to install Windows XP on his computer. In doing so he had to change Video cards and a friend gave him another monitor to try. Both monitor screens are green and it makes no difference to change video cards, the screen is still green. The windows flag colors show as Red, Green, Yellow and Black, the blue has dissappeared. The blue color gun on the monitor is black as well. Has anyone had this type of problem or know what may be wrong? We are wondering if it is in the AGP slot or the CPU itself. Thanks