This would happen if your normal.dot template has been saved as a mail merge
main document, or if it contained an AutoNew macro that converted all new
documents based on normal.dot to be mail merge main documents.

Suggestions:
a. ensure there are no such macros in your normal.dot (have a look at
Tools|Macro and take it from there - if there is an AutoNew macro, save its
contents to a text file in case you need them, delete the macro, close Word
and try again.
b. locate your normal.dot and open it. Then convert it back to a non-mail
merge main document (in Word 2000 or earlier, the Mailmerge toolbar should
be visible - click on the MailMergeHelper icon, click on the first button,
and choose the appropriate option. In Word 2003, display the mailmerge
toolbar via Tools|Customize, click on the first button and choose the
appropriate option, then hide the toolbar again. Then save and close
normal.dot

Peter Jamieson