A. According to Microsoft Office Online, there are several reasons why you might be getting this message in Word:

1. Your Word program might not be activated. To check, on the Help menu, click Activate Product. If Word is activated, you will see a message telling you that the product has already been activated. If Word is not activated, the Office Activation Wizard appears to guide you through the activation process. Note: You can open a Word document even if the program is not activated, but you cannot use many commands and you cannot save the document.
2. The document might be locked for editing or password-protected by another person as a security measure. On the Tools menu, click Options, click on the Security tab. If the Read-only recommended check box has a checkmark, you can uncheck it and click OK. On this same tab, a password can be required to open the file and/or to edit the file.
3. The document may be open in Word Viewer, in which case you cannot edit but you can copy and paste text into another document such as Wordpad.
4. If Word was shut down improperly while the file was still open, you might get a message stating, "The document is locked for editing by another user."
5. "This modification is not allowed because the document is locked" means that the author has used the Protect Document features to restrict editing. Menu commands also might be unavailable, depending on the permissions that were granted to you by the document’s author. This feature is available under Tools—Protect Document, and the restrictions/exceptions are listed in the Task Pane on the right side of the screen.
   
   In Excel, either the entire worksheet or selected cells may be locked or password protected. To lock or protect cells, click on Format—Cells—Protection, and click in the Locked check box. Hidden is used for formulas. In order for these features to be active, you must then click on Tools—Protection, where you can give the worksheet an optional password and permissions for changes may be allowed. You can uncheck the check boxes to unlock the worksheet.
6. MS Office 2003 has a feature called Information Rights Management (IRM) that allows the author to create content with restricted permissions. Click on File—Permission and the type of access desired.