If iChat is Open then Quit it.
Open Activity Monitor from Application/Utilities
Check the list for iChatAgent (It does not have an icon but should appear higher up the list than the DOCK)
Quit it with the red Quit button and confirm Force Quit. (This is because iChatAgent does not quit when iChat is Quit - You can also wait it out)
Now go to Users/(your account)/Library/Preferences and delete com.apple.ichat.AIM.plist and com.apple.ichatAgent.plist and restart iChat.