Cant open excel attachment in Outlook

When trying to open an Excel attachment in Outlook, excel opens but no spreadsheet is seen. The message “the system cannot find the file specified” is displayed. The user can right-click the attachment, choose “Save As” and save it to the desktop or somewhere else, then open it from there with no problem. Any ideas? Our email server is Exchange 5.5, SP4 running in a Windows NT 4 domain. The workstation is running windows 2000 SP3 and logs into both the NT4 domain and a Novell NDS tree (Netware 5.1 server). Thanks.