The QuarkIinvisibles font is installed during the QuarkXPress 7 & 8 installations. It is used to display custom (invisible) special characters, such as the em space and flex space, when View > Invisibles (Command-I) is selected. In this case the QuarkInvisibles font is installed but does not load when QuarkXPress is launched. The failure in this case is due to a Permissions/Privileges issue for the QuarkXPress Fonts folder.
Give Read & Write access to Everyone on the Fonts folder at the following locations:
Hard Drive:Applications:QuarkXPress 8:Required Components:Fonts.
C:\Program Files\Quark\QuarkXPress 8\Required Components\Fonts