I am running win 7 64 bit and the free CCleaner.
On a monitor with 1600 x 900 resolution, CCleaner will not fill the screen. If I check the full screen button, I lose a small part of the right side of the page. (I lose the close window "X" and I lose part of the "run cleaner" button on the bottom right.) If I try to drag it to the left, it shrinks to part screen display, even though there is room on the left of the screen.
However, if I move the windows task bar from the left of my screen to the bottom, CCleaner will then take up the whole screen. When the task bar is on the left, CCleaner will not use all of the screen up to the edge of the task bar, so it's odd that CCleaner "thinks" the task bar is in the way when it's on the left.