Sounds like the Mac OS icon database got messed up. To fix it, open System Preferences, click Classic (ignore any messages you get about Classic not being able to start because Mac OS 9 wasn't found or more than one System Folder is available and you haven't selected one), click Advanced, then click Rebuild Desktop. Select all volumes in the dialog box that appears then click Rebuild. Once it finishes, you'll need to log out and back in OR force-quit the Finder for the change to take effect.