Computer (CRT) Screen affected by Magnet

Try giving the monitor's degauss function a few more consecutive shots, a few minutes apart, turning the monitor off between applications.

If that doesn't work, you can try to manually degauss the CRT, but thats sorta tricky. Personally, I use a mains-powered commercial tape demagnetizer, but most folks ain't gonna have one of those laying around. A strong conventional magnet ... you're gonna want one that weighs several ounces at least ... can be used. The proceedure is fairly simple, if tedious; with the CRT off, position the magnet at the center of the screen (place a thin cloth or the like between the screen and the magnet to prevent scratching the screen surface). Start making tiny full circles with the magnet, expanding the circumference slightly with each pass, trying to make both the rate of travel and the rate of expansion roughly the same from pass to pass, and keep your motions smooth. Slowly expand your spiral of circles untill they are well beyond - a couple feet is good - the physical boundaries of the screen. At that point, smoothly remove the magnet from the vicinity - get it several feet away. For a 17" screen, the process oughtta take 3 or 4 minutes at least. With the magnet and monitor well separated, turn on the monitor and see what you've got. If you're lucky, and if you did it correctly, it'll probably have worked. If not, it'll be real interesting to look at - in a sorta '60s Psychedelic way.

You could try the hard core approach.

Actually, that'll work just fine - ya wanna strap the magnets down real well, and ya wanna use the expanding spiral method, getting the drill well away from the monitor before releasing its trigger, and it may take a few passes, but it'll work.