repair, but not reinstall
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..