I have a Dell Latitude D600 I have green horizontal checkering. As I scroll on a page it goes completely green. While still being readable.
At start up a bunch of unreadable gibberish comes up in black & white. When I try Safe mode the screen is unreadable.
This problem is still there when I plug into another monitor.

This started I believe after my son clicked on an alert message that popped up on the screen.

Perhaps their is something wrong with the graphics card do you have an onboard one or dedicated one? It maybe worth trying to get a dedicated card for your PC. As for clicking a pop-up message I am not sure what damage it could have done as you say it show gibberish as you start it up I would think this is a hardware issue.

It is the vdeo card. More than its worth to fix