You probably have an out-of-date contextual menu plug-in that is the cause of the problem (the Stuffit Expander one is somewhat notorious for this). If the Finder is running, navigate to ~/Library/Contextual Menu Items and move it out of the folder (or the /Library equivalent if it has been installed there). Logout and back in again and see if the problem has gone. If not, repeat for any other CMIs you have installed.
If the Finder won't stay up long enough for you to do this, log out then back in again, only hold down the shift key to disable any third party software that is normally loaded. Then repeat the above.