To fix this, reinstall the system fonts, or move them back into their folder (/System/Library/Fonts). This includes the Helvetica font.
Some third-party applications may allow you to move system fonts. Usually, you should not move or delete Apple-installed fonts.
Note: This document does not apply to the Helvetica font found in the Mac OS 9 System Folder used by Classic.
Related documents
106417: "
Mac OS X: Font Locations and Their Purposes"