Which version of the OS do you have ?
iChat is "matched" to particular OS versions.
Broadly an OS version has an iChat version.
For example Leopard (OS X 10.5.x) has iChat 4.x.x
Lion (OS X 10.7) has iChat 6
More specifically iChat gets it's updates in some of the OS Updates (not everyone)
So by the time OS X 10.5 has got to 10.5.8 iChat has got to version 4.0.9
iChat 4.x.x will only work in OS X 10.5.x even then you have to have the specific Updates to the OS and iChat together
iChat 4.x.x cannot be used in Snow Leopard (Upgrade) or Tiger (Downgrade).
Reinstalling involves the Install Disks and the correct COMBO Update od the OS version you are on to "Update" the reinstalled version to the OS level.
Instructions for version 3.x.x through to iChat 5.x.x
iChat 6 in Lion (OS X 10.7.x) requires you to use the Apple Recovery process.
The same applies to Messages in OS X 10.8.x
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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