Are you talking about the "Paint" utility that's built into Windows 7? When you paste images into it, it simply adds them to the existing default image that you get when you start the program. If that default image is larger than the screen capture, then you'll end up with extra stuff in it.
To change the size of the default image, click the "Home" tab and the "Resize" option. I've found that if you open Paint, set the size, and close Paint again without saving the image, it changes the default size for the next time you open Paint. What I do is set this to 1 x 1 pixel so that anything that's pasted will "grow" the image to it's own size.
By the way, the Snipping Tool is also terribly handy for capturing stuff from the screen...