Font Installer overwrites the font files in the /system/fonts directory. To install a font you must have root access and S-OFF (for HTC devices). Some apps use their own font and cannot be overwritten by Font Installer.
Before attempting to install any fonts please make sure you backup your fonts. You should also have a current nandroid backup.
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To restore your font to the default system font press the menu button while inside Font Installer, click “Backup and Restore”, and restore the original fonts you had (assuming you made a font backup before changing fonts).