I agree that the messy text is the result of trying to squish 1440 pixels worth of resolution into a smaller window height, in order to be able to fit the Taskbar along the bottom of the screen. Instead of some more subtle scaling effect, the game basically deletes certain lines of pixels, and when one of those lines crosses some text, the result is what's in your screenshot. I see the same thing when I switch to windowed mode without changing the resolution first, on a Steam install.
If Windowed Borderless Gaming doesn't work for you, try editing options.ini to have the screen height you want. The file is in Documents\Electronic Arts\The Sims 3, and you're looking for a line that probably reads like this:
resolution = 2560 1440 0
If you change the 1440 to something like 1360, the height of the window will be shorter while the width stays the same, so you should be able to have a normally scaled game while still leaving room for the Taskbar. (It's a good idea to make a backup copy of this file before editing it, just in case the game doesn't react well to the change.)
I don't get dark screenshots in Bootcamp, although my Windows install is not exactly up to date. I've been meaning to take care of that for other reasons (maybe this weekend), so I can experiment a bit if it would help. What graphics card do you have? Mine is a Pro 560; I might have a different driver than you do, in which case the effects of fiddling with the options in the Radeon software might not be the same.
