Outlook 2003

I am having a problem with Outlook 2003 password protecting my e-mails. When I open Outlook up, it used to ask me for my password, so no one else could get into my e-mail. Now, it doesn’t do it anymore. I went to the Personal Folders, right clicked and clicked on Personal Folders properties (to change the password) and clicked on the “advanced” tab, but got this error message “The operation failed. An object could not be found.”. Help! I want to correct this to go back to the way it was. Thank you very much.