If you see this message at the bottom of the application window when you try to modify a document: "This modification is not allowed because the document is locked," the document is protected with an editing restriction of No changes (Read only) or Comments in Word 2003 (or a restriction of Comments in Word 2002 and Word 2000).The document author 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 author.
Answers & Comments
This should only happen if you are using a trial version of Word that has now expired.
Do you have a full, registered copy of Word?
If you see this message at the bottom of the application window when you try to modify a document: "This modification is not allowed because the document is locked," the document is protected with an editing restriction of No changes (Read only) or Comments in Word 2003 (or a restriction of Comments in Word 2002 and Word 2000).The document author 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 author.
Via: http://office.microsoft.com/en-us/word/HA011406121033.aspx