To do that press the Print Screen key and then go into an image editing program (like Windows Paint) and paste the clipboard to an image.You can also use free programs like Fraps to write screenshots directly to a file.
A lot of hardware must be functioning properly to generate images correctly but there are three main sources of problems: the video RAM on your video card, the GPU which is the main chip on the video card which does the drawing, and the bus interface which transfers data between your video card and your motherboard.
There are instructions on how to underclock your video card on this page.
And if you're overclocking your video card then you should back off on your overclock. Excessive overclocks can raise the temperature enough to cause cumulative damage.
If you see artifacts during the power-up screens before your operating system loads then you know it has nothing to do with drivers. If you're seeing visual artifacts in just one program then it may be a software problem with that program.
But if you have artifacts in many programs then you may have a driver problem or bad hardware.
When RAM goes bad, quite often just a small fraction of the RAM chip fails while most of it continues to work properly.