Problem with displaying full hd resolution when hdtv is used as a monitor

First of all, I would like to greet all the members, and to apologize for writing such a long post, but I have a great problem. I don't know if anyone mentioned it, I really couldn't browse through 400 pages this time, so here goes...
I have recently bought Samsung HD Smart TV and when I connected it to my PC via HDMI, it worked like a charm. I had Windows XP. However, I wanted to try windows 7, so I installed it, and the nightmare began. Full HD resolution was never displayed anymore, and, depending on a movie, screen was more or less pixelated, even though they are supposed to be 1080p. Sometimes there is flickering. And today, I saw that even the pictures from my PC and Facebook also became slightly pixelated.
Several ordinary DVD Rips I have looked like VHS rips. Now, every movie is pixelated and the picture is not as crisp as it used to be, by a long shot...when my friend plays a movie on his laptop, the picture is crystal clear.
Any ideas? Is the problem in PC or TV? I tried virtually everything, installing the drivers at least 50 times, so as Windows 7 itself. I tried switching back to XP, to no avail.Scaling, pixel formatting, changing the label of the hdmi input, preventing windows from installing drivers on its own...I tried it all...I don't know what to do, I just want the quality that I saw when I bought the TV for the first time.
Sorry for the confusing post. Cheers.

By the way, my video card is ATI Radeon HD 4650. I run Windows 7 /64 bit. 4 GB Ram. 2,3 Triple Core Processor.
 
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This should be a 1080p rip of The Terminator, but it isn't. Color depth was set at 32bit.
 
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