I don't know of any specific solution, but you might just have to trash the iChat plist.
hmm where can I find this? I'd love to solve this problem.
Open your user directory (Home)>Library>Preferences folder.
Scroll down and move the com.apple.iChat.plist to your desktop.
Restart iChat and see if that solves your problem. If so, delete the plist file from your desktop