You probably have one of two things wrong. A) Your resolution got messed up or B) you it somehow got magnified by the operating system (I think this would only be a problem on Vista and Win. 7 machines).

To solve:
A) Open the control panel > personalization > resolution and move the slider or drop-down menu for Vista and Win. 7. For XP, do as Titus said and right click the desktop, select properties, open the settings tab and adjust from there.

B) Open up personalization from the control panel and select magnification I believe. Change that however you like.

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