Now it's 1600X1200 and when I click on “duplicate”, Windows changes it to 1440X900.
Actually,
your Win7 video shows that this is NOT what happens when you click on "duplicate". It was YOU who changed it to 1440x900, not Windows.
What actually happened when you clicked "duplicate" was that for some reason Windows changed the resolutions to 1280x960... from the original 1600x1200 which was originally in effect on the CRT monitor. Note that 1280x960 is also a 4:3 aspect ratio resolution, similar to 1600x1200 which is also 4:3.
Windows made that resolution change when you initiated "clone" mode because 1600x1200 cannot be displayed on the 1920x1080 Vizio screen. Maximum vertical size is 1080, so 1200 is too large. Remember it's a 16x9 HDTV, not an actual computer monitor which might support additional VESA resolutions. 1200 vertical is unacceptable, since the Vizio only has 1080 vertical maximum (remember, it's a 1920x1080 screen).
So Windows reduced the "clone" resolution down from 1600x1200 (4x3) to 1280x960, which is also a 4:3 resolution and is acceptable to the Vizio's vertical screen size. In fact it is apparently the largest possible 4:3 resolution the Vizio can accept (in its maximum 1080 vertical screen dimension).
And judging from other the values shown in the slider when you decided to dropdown the list, in addition to 1280x960 the Vizio can also accept 1280x1024, 1360x768, 1360x1024, and 1440x900... all of which do not exceed the Vizio's maximum of 1080 vertical screen size dimension, while at the same time also being acceptable to the Lenovo 4:3 monitor.
Note that as shown in
your Win7 Res Change video, when you did manually (yourself) change to 1440x900, this image was presented inside the Vizio's 1920x1080 screen size as a 1440x900 centered image. So it looks "postage stamp", i.e. horizontally and vertically centered at 1440x900 size within 1920x1080 real estate, and technically speaking there are black bars on left and right, as well as on top and bottom... hence "postage stamp".
Similarly, if you'd left things the way they were originally changed to when you pushed the "duplicate" button (i.e. when Windows changed it to 1280x960), you would have again seen "postage stamp" on the Vizio, displaying a 1280x960 window within 1920x1080 real estate. Of course the 1280x960 (4x3) postage stamp would have been shaped differently than the 1440x900 (16x10) postage stamp, but both would have been "postage stamp" within the 1920x1080 screen size of the Vizio.
Again, the Vizio is an HDTV (not a true monitor) and has a set of acceptable display resolutions it can handle, up to a maximum of its "native" recommended resolution of 1920x1080 which would therefore fill 100% of its screen real estate with a 1920x1080 Windows desktop image.
It is unchecked in Windows 7. If I check it there things get ugly.
I didn't see a visual proof of this statement in Win7, in any video. You showed clone mode details and that check box for XP
in your WinXP video where the box IS CHECKED for both monitor and HDTV.
But you didn't show the equivalent in Win7, which you now say IS UN-CHECKED!! This is the opposite of how you have things in XP as clearly proven by your WinXP video, so now I'm not entirely surprised that things don't work exactly the same way in XP and 7.
So... how about CHECKING the boxes in Win7, to match how you have it in WinXP?? You say "if I check it there things get ugly", but I'd like to see what you mean... in another video please??
Anyway, I really do want you to upgrade your Catalyst driver to
the latest 13.9 LEGACY driver, along with the matching current Catalyst Control Center. You don't need the optional HydraVision package at the bottom of the page. When you get the 13.9 components successfully installed, be sure to go into Catalyst Control Center and switch to "advanced mode" from the default "basic mode", so that you can see all options for everything.
And one more thing. If you look at
your XP Desk TV video, you'll notice that on the left edge of your WinXP desktop on the Lenovo monitor there are 15 rows of shortcut icons, with the top row having 5 and the bottom row having 1. Now look at the Vizio screen later in the same video. Note that there are only 12 rows of shortcut icons, with the top row having 3 and the bottom row having 2. So the Vizio presentation in "clone" mode on XP has chopped off the top and bottom of what is simultaneously visible on the lenovo monitor. I don't know if this is being caused by some ZOOM MODE you might have active at this moment on the Vizio HDTV (which seems likely to me as the only plausible explanation), or some other additional oddity anomaly in the XP Catalyst driver and "clone" mode with different screen sizes.
. Hopefully you get it.
I think I understand it based on plain English explanation 
BUT (don't kill me for that question) How is that possible to still have 1600X1200 resolution on the monitor in Windows XP with the same exact setup? I mean everything is the same except the Windows version. Same monitor, same TV, same video card, same cables, same mouse pad 
Somehow those rules could be overwritten in previous versions of Windows. I want to know how and I want to overwrite them again. This is why I'm here asking for help. As a matter of fact before I posted here I've done a lot of research on the web and people like me (looking for the same solution) were saying that it was also possible in Vista. I've never used that OS so I can't vouch for that. Went to XP from 98 and would run it for another ?? years if Microsoft would support it.
