Opening Oulook 2000 File Attachments

I used to be able open file attachments that came with E-mails in Outlook 2000, but now when I click on an attachment to open it I get various error messaages such as, “The server, application, source file, or file item cannot be found.” This applies to PDF files and text files. Something is stoping outlook from open E-mail attachments. What can I do to trouble shoot this?