How can i change my Microsoft Word default document format?

Microsoft uses a file named normal.dot (document template) to hold the default settings for documents created using Word. If you want to change those settings you need to open normal.dot, make your changes and save the file. Where normal.dot is stored depends on the version of Windows and Word you are using. Assuming you are using Windows XP, when opening the file navigate to the C:\Documents and Settings\user_name\Application Data\Microsoft\Templates folder. Be sure to change "user_name" to the ID that was created when Windows was installed.

Changes have been made that were probably marked as Default by mistake. Do a search for normal.dot and delete it. When you restart WORD, it should create a new normal.dot based on standard default.