Has it ever worked properly before?
Is this a laptop?
Did something get changed and then this suddenly started happening or
did it happen gradually?
If you didn't change anything to get it this way then I would not just
start changing settings at random since no change you can recall got
it here in the first place.
Is it only on the desktop or does the screen not look right when you
run other programs? Does it look normal when it boots or are the
colors messed up there? How about if you look at some WWW page, or
your email program?
For a test of just loading the basic video drivers, try to boot in
safe mode by pressing F8, F8, F8... during the startup and choosing
safe mode. Do the colors look better (but everything bigger).
Maybe it is a bad/loose video connection or cable.
My monitor has a cable that runs from it to the output on the
motherboard. I can take the video cable off from both ends. I would
try removing the cable completely and make sure all the pins are
straight and then attach it again making sure it is secure - not just
plugged in hanging there, screw it down. If you are using a video
card, remove it and reseat it even in a different slot if possible, no
power at the time of course.
If you jiggle the cable around (especially near the ends) does it look
good or flicker while you are jiggling (the cable) for a while?
Can you try another known good computer with your monitor
temporarily?
Can you try another known good monitor with your computer temporarily?
Can you try a known good cable temporarily?
A lot of this will not be easy/possible to try if you are using a
laptop - you didn't say. If it is a laptop, it probably has a video
output you can hook to an external monitor and see what happens.
If all else fails, you might give your monitor a good whack if it is
an old CRT model. Used to fix my old TV for a while. Or just whack
it anyway.