I see this Post appears to have been moved from the Mac Pro area.
iChat 3 though 5 can be reinstalled via the Install disk by the methods described here in this Users Tip.
The so called "easy way" requires inserting the disk and then expanding the window for the Disk so you can see the Optional Installs folder and then running the OptionsInstalls.mpkg
(Depending on version that might be another folder deep)
From there you run the Installer and pare down to just installing iChat.
You will then NEED to Download and Install the COMBO version of the Last Update your did.
This will be needed to Update iChat to "Match " the OS.
The "Hard Way" is just complicated rather than difficult and uses Pacifist to extract the apps from the Install Disk.
This too needs the COMBO to be run afterwards.
A Time Machine Back up maybe easier for Lion (iChat 6).
I have not used this but you are supposed to be able to Restore from the Drive
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously