Antony's guesses are rarely wild.
Either:
- Start > Control Panel > Display; or
- right click on an open area on the Desktop > Properties.
Then: > (tab) Appearance > (bottom item) Font size: .
This is a 'universal' font size control that establishes the size for Windows-controlled* texts.
You can fine tune these by clicking on the 'Advanced' button, then experiment to adjust the various types/areas of displays.
Windows-controlled* - Applications may use the 'Windows-wide-default' font characteristics, which are stored in the Registry.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.11) Gecko/20071127 Firefox/2.0.0.11