David upgraded to Windows 10 and now he can't open any spreadsheet attachment without entering a key. Leo says it sounds like Outlook is blocking the opening of attachments to protect him. This is because of ransomware, which has been triggered by opening an attachment. Ransomware, once opened, encrypts all user data and then asks for money to unlock it. So it's a security feature that it doesn't let him open those attachments. He may be able to disable that in settings, but the IT Department where he works probably has that enabled for a reason.