You didn't try using "macbookprompls" when typing a command in Terminal.app to locate these plist files, did you? "macbookprompls" was the original poster's screen name, which I used by example to imply that (s)he needed to use his(her) short user name. So, if you were, you should be opening up, in the Finder, your user home account's Library's Preferences folder. There should be a several files in there whose names begin with "com.apple.iChat" and end with ".plist" and if you quit iChat and trash all of those files and restart iChat, it will look like a "first time ever" launch of iChat on your computer. Then you should be able to recreate any iChat, Jabber, and/or Bonjour accounts with your ichat, jabber, and/or bonjour screen names, and all should be well. Things like AIM buddy lists are maintained on AIM servers, so you won't lose those by deleting these plist files.
If that is of no help to you, I
think that with the Leopard installer DVD, you can do custom reinstallations of a limited number of Apple OEM applications, iChat being one of them. My installer DVD is not conveniently nearby for me to check, but that was the case with Tiger. Some info for custom reinstalls in Tiger can be found
here. I can't find an equivalent knowledgebase article for Leopard, though. In any event, if iChat is one of the apps that the optional installs package on the Leopard installer DVD supports, you could try a reinstallation of iChat from the optional installs package on the Leopard installer DVD.
However, before going down the optional installs route (or maybe after, too), if you launch iChat from the command line with error logging, and post the log results here, the gurus here in this forum could help you troubleshoot further. To launch iChat from the command line, start up Terminal.app (in /Applications/Utilities) and type
/Applications/iChat.app/Contents/MacOS/iChat -errorLogLevel 7⏎
and copy and paste the resulting text displayed in the Terminal window back into a follow-up post here, if iChat still refuses to launch for you.
