Hmm... never had this problem before, but I'd recommend the following if you still have Trillian installed:
Close out of Trillian and then delete your aim.ini file located in your Trillian\user\default directory. The default directory is if you only have 1 account using Trillian. Once you have that file deleted, restart Trillian and it'll recreate the file for you.
If you don't have the program installed, I'd recommend deleting the whole Trillian directory and then reinstall the program.
The default install path for Trillian is: "C:\Program Files\Trillian\" Unless you made changes to where it gets installed.