You could repair it, depending on what the problem is, but you can't reinstall just iChat as iChat is directly part of Jag, rather then a seperate app.
I can't tell you how to repair it right now cause I'm at my school where they are still running OS 9 so I cant look at what I'm telling you to do, but if you post the spacific problem I'll be able to try to help you repair it when I get home, if you want and if iChat isn't completly broken you can IM me at VHFipher, well Resors Room is over, off to Physicall Education..
actually, if you pick up pacifist on vt, you can dig through the packages on your jag cd (no.2 i believe) and just install ichat. pacifist is a great little app for stuff like this. i've had several errors with ichat that looked like that and i think it's just the fact that ichat was rushed. does this bug prevent you from doing something? what exactly does it affect?
If you can't do it his way, wich is very cool and interesting, then try trashing the prefrences and then restarting the computer. Trashing the prefs for an app can fix the app in many situations. The prefs are located at:
They are called:
"com.apple.iChat.plist", "com.apple.iChat.SubNet.plist", and "com.apple.iChat.AIM.plist"