I'm using Windows 7 64 bit and a Logitech G400 mouse. When in windowed mode, the game seems fine, other than being so small that I can't read the writing even if I strain my eyes. In fullscreen mode the mouse suddenly is complete crap. It has this completely retarded acceleration that it's never had in any other game or application, so it's clearly a CC issue. When I move the mouse slowly, the pointer will barely move. But if I move it a little too quick, it will shoot across the screen. It's just an insane amount of acceleration and it makes the game completely unplayable in fullscreen mode. It's not simply annoying or uncomfortable; it's so bad that I spend the entire time struggling to move the pointer to where I want it to go. If I move too "fast", the mouse suddenly shoots away from the destination across the entire screen instantly, but if I move ever so slightly slower, it's like it's moving 1 pixel per hour, which means I need to spend a long time crawling back to the destination if I accidentally move the mouse too "fast". Turning the mouse sensitivity down fixes the issue of missing the target and, instead, means I have to move the mouse a few dozen metres to get the pointer to move anywhere. Is there a way to reduce the acceleration, perhaps to the perfectly good level that windowed mode has?

Also, in an attempt to get around this I tried to use the 2x window mode, but all that happened is the window went black, then came back with picture, but at the exact same size.

Please, does anyone know how to fix this? The game is unplayable either way; either I can't see what's happening nor read anything, or I can't use the mouse because it's just impossible.

I have the same problem, same specs (g500 mouse, though) and find that I need to move my mouse in ridiculous patterns to go anywhere, 2x also does not work for me. In fullscreen it's a nightmare of slowness, and in windowed, not only is it too small but I have to wave the mouse like a magic wand to move it. This doesn't seem to be an issue for b26, just, apparently, b27.
I got it from the HIB 3, not from key purchase.