By putting ATI in Google, I find that they seem to be an AMD subsidiary or "associate." Clicking on Support at the AMD site, and asking to "troubleshoot a display problem," using as much as you've told us about your computer and the problem gets:
The suggestion is that you check for the current driver for your setup. Downloading and installing a new driver should be fairly trivial, and something you should be able to do easily if you read and follow the instructions.
There is also some comment relative to:
"Missing textures, scrambled video display, flashing areas, or a solid colored screen are common symptoms of graphics corruption. This is usually the result of the game using a graphics feature that is not supported by the video card (graphics adapter), an issue with the game, or a display driver issue.
"Please Note:
"Running any game on systems that do not meet the minimum requirements may result in instability, graphics corruption, and/or severe performance problems. Please check with the game developer/publisher to ensure that your system meets the minimum requirements of the game. "
Given the (lack of) quality in much "entertainment media" distribution, you probably can substitute "movie" for "game" in the above, although if you do have a couple of games on your computer you should check with the "Review the Games Knowledgebase section to see if this is a known issue with this particular game."
If the problem is being caused by a game (or movie) that's using "features not supported" by your computer hardware, there's nothing ATI can do about it except tell you to quit using the game/movie and/or upgrade your hardware. That would be in the "Games Knowledgebase" section. They appear to have responded to this problem(?).
There are separate instructions for Vista and Windows, so you'll have to scroll down quite a distance to get to the Windows section. You may run into complexities that you will prefer your "computer shop" to handle if you get much past downloading a driver update, but you should be able to handle things at least through that point.
I did scan several of the whinge sites that complain about this being an "unsolvable problem" but in the ones I looked at there was NO MENTION of anyone checking with ATI to see if there was an updated driver - - but maybe I just missed it(?). The page I linked tells you how to tell what driver you are using, and you can easily look to see if there's a better one to use.
There is the possibility that you also have a "hardware failure," most likely in your "grapics card." The troubleshooting procedures can give some help with identifying whether this may be likely.