I think your normal.dot (template) has become corrupt.
Normal.dot is so much more than a template and seems to control from 'open' onwards.
In earlier versions it was easy to find this file in MS Word folder.
The principle is still the same, find it, delete it or rename it, and next time Word opens it will make a brand new Normal.dot.The only difference will be a default version which undoes any personal changes you may have made in th past
I have Office XP, but use it with XP Pro. It may be different in Vista.
If you follow this path in My Computer you might find it!
C:\Documents and settings\"Chasg"\Application Data\Microsoft\Templates\