Screen appearance problem

Question

I searched the forum before posting this question, but may have missed an on-topic post with my screen appearance problem.
While using the computer, the screen appearance abruptly changed: the image was magnified so much that it does not fit the screen. I tried using system preferences to change the display settings, but if I shrink the setting then not all the screen is displayed; there are ugly random colors in a square block pattern on the edges.
I have only had the computer a few days, the OS is the Snow Leopard, but I couldn't recall (or find written) the OS number. It's the smaller of the new Imac screens, 21-inches I think, not the 27-inch.

Answer 1

Welcome to the Apple Discussions.
Have you tried to reset the PRAM and reset the SMC? These actions may resolve your abrupt problem. If not you certainly want to call Apple as it may be a serious graphics issue.
To reset the PRAM, shutdown the computer, press the start button, as soon as you hear the boot chime, hold down the COMMAND OPTION P R keys until you hear two more boot chimes, then release the keys. That may be all you need to do.
To reset the SMC, shutdown the computer, unplug all power to the computer, press and hold the start button for at least five seconds, plug the power cord back into the iMac, press the power button normally to boot up.
If neither of these procedures help. you can execute the Apple Hardware Test, by inserting your Applications Install DVD, restart the computer and hold down the D key until you have booted. This will boot you right into the Apple Hardware Test program. That might show you that you have a bad braghics card or some bad memory, etc. You'll need to call Apple then.