sjph,
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Are you describing the font on the Desktop rather than on the home page in a web browser? If the problem is with the home page in a web browser, open the browser's preferences and reset the default font.
If the font is wrong on the Desktop, that's a more serious issue since the default LucidaGRande font should never be moved or changed. If you open Applictions -> Font Book, click on "Computer" in the windowpane on the left, scroll down to Lucida Grande, and click on that name, does it appear normal? If you open a text document, type something, and change the font to Lucida Grande, does that also change to the missing character symbol? With LucidaGrande selected in FontBook, select Validate Font from the File menu and see if that reports any issues.
According to the invaluable
Kurt Lang – Font Management, garbled fonts are most often caused by corrupted font caches. Using a third-party utility to clear font caches should help in that case. You can clean font caches with
Font Finagler or
OnyX. Note: if you use OnyX (or Cocktail or a similar multi-purpose utility), be careful not to get carried away with all the options for clearing various caches! Clear only those that may be involved in a specific problem.
