Although it doesn't look like it, the Activity Monitor uses the Internet Explorer engine to display the information.
Do you use IE as your main browser, or are you using something else (like Firefox, Chrome, Safari, etc)?
You might want to try opening IE, and resizing the window to fill most of the middle portion of the screen (but NOT maximized, and not filling the entire screen).
Then, close IE, and try opening the Activity Monitor again. I don't know if it will help, but I've had this fix work for other apps that use the IE engine.