See http://word.mvps.org/FAQs/AppErrors/BlankDocNotBlank.htm .

hi,
This problem happens when you have made changes to the Normal.dot template.

The Normal template (Normal template: A global template that you can use for
any type of document. You can modify this template to change the default
document formatting or content.) opens whenever you start Word, and it
includes default styles, AutoText, macros, toolbars, and other customizations
that determine the basic look of your document.

On the File menu, click Open, and then navigate to C:\Documents and
Settings\user name\Application Data\Microsoft\Templates.
If no templates are listed in the Open dialog box, click the arrow next to
the Files of type box, and then click Document Templates.

Double-click the Normal.dot file to open it. To be certain that you're
working in the default template, check to see that Normal.dot appears in the
Word title bar.
Make any changes you want, using the menus and dialog boxes just as you
would to change default settings for a document, but remember that any
changes you make to Normal.dot will be applied to documents that you create
in the future.
When you have finished, on the Standard toolbar (toolbar: A bar with buttons
and options that you use to carry out commands. To display a toolbar, click
Customize on the Tools menu, and then click the Toolbars tab.), click Save .

Note  If Normal.dot is renamed, damaged, or moved, Word automatically
creates a new version (which uses the original default settings) the next
time that you start Word. The new version will not include any customizations
you made to the version that you renamed or moved.
hope this helps, it has happened to me too!  jodona50