By changing your Normal.dot (that's the general template Word uses to create
any new document for you). Go to C:\Documents and Settings\user
name\Application Data\Microsoft\Templates. You may want to take a copy of
"Normal.dot" before doing anything to it. Then right click on Normal.dot.
Select "Open" and modify the style "Normal" to what you want it to be. (This
is the underlying text style for all new documents and most other text
styles).
A text style saves all text formatting that you want to include by default.
To modify (or create) styles, select "Format - Styles and Formatting" (in
Word 2002/03) which will open the Styles and Formatting Panel on the right.
In there find "Normal" and select "modify" on the dropdown menu (arrow
appears on the right when you hover on it). In the dialogue, change the font
settings to what you'd like it to be and go OK.
!!!Please be aware that other templates may be based on the standard
Microsoft Normal Style and changing Normal style may impact documents that
rely on that style (i.e. everything called Normal that looked liked Times New
Roman so far may well be changed now)!!!
Generally, it's not best practice to change the inbuild Microsoft styles but
rather build your own ones or create style aliases.
Have a look here:
http://office.microsoft.com/en-us/word/HP011210281033.aspx