OS X 10.5 (Leopard) would have iChat 4 on the Computer already.
If you deleted it then it can be "reinstalled" from the Install Disk.
Put the Install disk in the computer
Drag the new window wide enough to see the Optional Installs folder
In there open/launch the Optional Install Package.
It will look like a normal installer at first.
As you progress it will offer a list of items in Application and Printers Drivers.
Tick iChat only and then continue.
BEFORE doing anything more Download and Install the Combo Version of the last OS update you did.
http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update (Latest version)
This will update the Installed version of iChat from the version at the Install Disk Level to the latest version.
The iChat version HAS to MATCH the OS version they are being used with.
However judging by the message I am not entirely sure you are using Leopard.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously