This answer in our Support FAQ covers MissKathy's answer above and adds a bit more:

The game's display size is 1024x768 pixels, so if you are attempting to run it on a display with a resolution that is less than that (such as 640x480), the game will not fit. If your display resolution is exactly 1024x768, you may need to try changing the setting of fullscreen mode (on the Options screen).

To check and/or change your display resolution for Windows XP, 98 or ME, right-click on an empty area of the desktop and select properties from the popup menu, then click the Settings tab.

For Windows Vista, right-click on an empty area of the desktop and select personalize from the popup menu, then click on Display Settings at the bottom of the list. The current screen resolution is displayed and may be adjusted by moving the slider.

For Windows 7, right-click on an empty area of the desktop and select Screen resolution from the popup menu, then set your screen resolution.