First, figure out what style is applied to tables by default. This depends on the version of Word as well as what customizations (if any) have been made; on my computer, it's "Table Grid". Then, open your default template (normal.dot) and modify that style to apply a .08" indent to the entire table (under Table Properties).
Note that the .08" measurement is the default cell margin. If you've changed that to a different number, make sure your table indent matches the changed value.
Note also that this only fixes the left margin of tables. I don't know of a way to automatically affect the right margin as well.