This occurs if you are using the Clear Type font smoothing method that was introduced with Windows XP (and which is based on the questionable assumption that making the fonts more blurry will make them look smoother).
To change back to the standard method of font smoothing, right-click anywhere on the desktop. The Display Properties dialog appears.
Select the Appearance tab, as shown above, then click on the Effects button to display the Effects dialog.
In the setting Use the following method to smooth edges of screen fonts make sure to select Standard, and not Clear Type. Click OK as many time as needed to close the succession of dialogs that you have opened.