Ok, so I was having the same problem as everyone else, with cs4 and 5, the document window always opened in my tools window. I would have to drag the image to my second monitor which was a pain. Everyone said check your drivers vid. card and whatnot. So while meditating with some wine, I got to thinking that the problem started with cs4 which is the first version where you could dock windows together. And when I opened an image, it always docked with the main window.
This is how I fixed it:
Edit>preferences>interface...uncheck open documents as tabs!