Add to your stylesheet
Code:
#loginDefault.centerColumn {
height: 584px;
width: 543px;
overflow: auto;
}
However, the only thing this conceals is the submit button, which could confuse people. You might want to try tightening up the layout.
The scroll area has to be narrower than the page to keep from cutting off the logo. I also notice that you have the left and right columns active, apparently used as spacers to keep the center column in the main background image. A better way to do that would be to turn off left & right in Configuration > Layout Settings, and make #mainWrapper the width you want, with "margin: auto;", in the stylesheet.