Let's check the Internet Control Panel first. Open it then click on the Advanced Tab and select File Mapping from the list of icons on the left side of the window. Click the Extension column label to sort the File Extension Mappings list by file extension then look for the .bin entry. It should have StuffIt Expander showing as the Application. If some other program (PowerPoint?) is assigned click the Change button then find Expander from the list of programs you will be given. Close the Internet Control Panel window and save the changes.
In your web browser you will find something similar in its Preferences. In Netscape it is under the Navigator category in the Applications entry. Internet Explorer has it under the Receiving Files section in the File Helpers entry. Scroll through the lists looking for .bin and/or PowerPoint entries until you find where it is assigning them to each other and change the entry so that Expander is used instead.
Note that this definition can happen in either the program or the OS or in both so check each carefully.