If the message is "This modification is not allowed because the selection is
locked" then see if any of this helps ...

If the problem is happening in just one document then it could be that the
document is protected or marked as Final.

To check for protection, on the Developer tab, click "Protect Document" and
from the menu that appears, click "Restrict Formatting and Editing". If there
is a "Stop Protection" near the bottom of the "Restrict Formatting and
Editing" task pane, the document is protected.

To check whether the document is marked as final, click Office Button >
Prepare and check whether "Mark as Final" is selected (in my case its icon
shows as surrounded by a light orange box).

If the problem is happening with documents that you are trying to create
from scratch then you might have a problem with your Normal.dotm file and you
may need to rename that.

If the problem is happening with all documents then the following might be
relevant.

Some postings suggest that you might get this if you're using a trial copy
of Word and the trial period has expired, or if you've never activated the
product.

There's this from garfield-n-odie:

===========================================
What happens if you start Word and click on Help | Activate Product?  If
you are already activated, a dialog box saying so will appear.  If you
are not activated, the Activation Wizard will start.  If you have an
expired trial version, a dialog box will prompt you to enter a valid
product key or convert (purchase a key) online.
===========================================

There's also this (slightly modified) from a posting by Bob Buckland:

===========================================
"There are a couple of scenarios that can cause this.  One of the most
common is a corrupted activation file, another is the software not 'passing'
one of the MS Genuine Advantage checks for Windows or Office (and those
aren't always correct <g>).

With all of the Office apps closed and in a profile that has administrator
privileges, use Start=>Search to search for the file opa12.dat  and rename it
to opa12.old, then restart the PC and open Word to see if you can activate
Office 2007."
===========================================

If none of this helps, post back and someone may be able to suggest
something else.

Regards.

Ed