First try repairing permissions:
Applications>Utilities>Disk Utility>Select your harddrive on the left window then click "repair permissions". You might want to restart after doing this but im not sure if its necessary.
If that doesn't work try trashing your iChat preferences:
(Note that this will reset all your iChat preferences)
UserFolder>Library>Preferences> My computer has 3 preferences associated with iChat: com.apple.ichat.aim.plist, com.apple.ichat.plist, and com.apple.ichatagent.plist. Delete these files and then try to start iChat.
Remember, this will revert iChat to its default preferences, it will be like running it for the first time.