Occasionally certain fonts in your computer's font library will go corrupt, causing various printouts or interfaces to display incorrectly. See the attached "Printout Fonts.pdf" for a map of which fonts are used in default printouts. This will help you determine which font has been corrupted or missing. Once you've determined which font is corrupt, remove it and re-install. Use a font utility to validate all fonts and check for duplicates.
See these external web sources for additional information:
Mac: http://support.apple.com/kb/HT2509?viewlocale=en_US&locale=en_US
PC: http://windows.microsoft.com/en-US/windows7/Install-or-delete-fonts