May sound dumb but have you tried holding Ctrl and scrolling the middle mouse back? if not turn off then on again. You may have the helper thing enabled for disabled people, which zooms it in. However I'm unsure of how to turn it off.
In the Display Window, click the Settings tab
Adjust your screen resolution.
If this is not an option because it is greyed out, it means your video drivers probably have failed or become unavailable.
(This is the only solution I can think of at the moment)
If this doesn't work, try restarting your computer.
EDIT:
(Copied from some Microsoft help forum):
Simply restart the computer. If this doesn't work, try doing the following: Type in Resolution in the Start Menu and select "Make text or other items larger or smaller". Choose 100% if it isn't already chosen. If this doesn't help, type Resolution again and click "Adjust screen resolution". Now, choose the reccomended size. If that doesn't do it, choose a smaller than reccomended size. If that still doesn't do it, restore your computer to an earlier time.