You've not told us which version of Windows you're using (assuming that you're even using Windows rather than say Linux or an Apple Mac). It makes a difference!
This is for Windows 7 and might still apply elsewhere:
Go to Start > Control Panel
Click on Appearance and Personalization.
Then go to 'Make Text & Other Items Larger or Smaller' (under 'Display').
Click next to 'Smaller (100%)' and then on Apply.
Note: If 'Smaller' is already selected it suggests that your computer has reduced its screen resolution. Click 'Adjust Resolution' (at the top left of that page). Click in the Resolution field, move the slider up and then click on 'Apply'. (Don't panic when your screen goes blank for a few seconds; that's normal!)
If 'Smaller' is already selected AND your screen resolution is already set to maximum then it suggests that you might need to reinstall your computer's video driver. (We'd need the exact make and model to point you to the download).