MAJOR Font Problem

Before you reinstall the fonts, try using Font Book to Resolve Duplicates (Edit menu). Do you have any theme changers installed?

 

if'n it's only happening in your User, then in must be specific to something you have in your own user folder, likely in your account's ~User/Library/Fonts

 

This will happen on occasion in Mail and when I browse in Safari. I spent a couple of days searching it down and found that (and was confirmed by online research) that older postscript fonts and some True-Type fonts will cause the application to use fonts the system doesn't recognize. I once had "Mail" using "Helvetica Fractions" for it's display font and the only way to resolve the issue was to delete it off the machine totally and re-boot.

There is a lot of information out there on how to remove or handle unwanted fonts in OS X, try a Google search like "Deleting fonts in OS X" and you can find tons of help and reading.

But if you haven't already heard, NEVER, and I mean NEVER remove "LucidaGrande.dfont" from the System>Library>Fonts or the machine will never boot up.