If you turn off (disable) the Helvetica font (the Apple-created one) on your computer, you won't be able to start an iChat chat. Instead, you'll see some gibberish message, like this:
iChat Internal Error! NSInvalidArgumentException: NSConcreteMutableAttributedString addAttribute: value: range:: nil value
What's up with that?
iChat needs the "Helvetica" font included with Mac OS X to start a chat (you may have other "Helvetica"-type fonts installed with similar names). It's easy to turn the font back on if you used Font Book to turn it off. If you disabled the font using another third-party font management application, use that software to turn it back on, or disable the software.
This document applies to Mac OS X 10.3.9 and earlier.