There seems to be some confusion here. Did you buy that piece of equipment as a television or a PC monitor? If as a TV then there is nothing wrong with it at all.
If you bought it as a monitor for a PC then what you've seen so far is as good as it gets. It simply is not good enough to act as a PC monitor in any real sense.
Page 49 is telling you that with a 20inch TV and VGA input the best you can get is 640 x 480 at 60hz. The SVGA, XGA and SXGA input modes are only available on the 15 and 17 inch models so even if you had that output from your PC your TV couldn't cope with it.
Your next step? Use the Samsung for it's intended purpose, a 20 inch television and nothing else. If you want a 20 inch PC monitor I'm afraid you are going to have to go and buy one. Or, of course, live with what you've got.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.