Backup your message history. To do so please shut down ICQ ('Main' menu button ->Shut Down), now copy all files inside the directory ICQ2002a into a different location on your hard disk. (The '2002a' folder is by default under: C:/Program Files/ICQ2002a).
Run the ICQ Database Converter feature - Aimed to fix problems in your local database which contains your message history and contact list information. While ICQ is shut down, click the 'START' button -> Programs -> ICQ -> ICQ Database Converter.
In the 'ICQ Owner' field choose the ICQ number, in the 'History Options' field choose the history range you wish to keep and click "Next" After the conversion is completed, click the "Done" button.
http://web.icq.com/help/faq/1,,1732,00.html#gs3