Since this is heading numbering, you should definitely make use of
multilevel (outline) numbering.

The quick way is to use Home tab | Multilevel List | Define New Multilevel
List. You'll see a dialog box similar to the Customize Outline Numbered
dialog of previous versions, where you can specify the options for
numbering, attach styles, and so on (for more on the dialog box, see
http://www.shaunakelly.com/word/numbering/OutlineNumbering.html).

To edit such an outline list (as well as most lists created with Word 2003),
you would do this: Place the insertion point in a paragraph to which the
top-level heading style has been applied (such as Heading 1 for lists
created with the built-in headings), and then click Home tab | Multilevel
List | Define New Multilevel List. Note that you may have to use Ctrl+Q to
reset the numbering of existing paragraphs after you've modified the
numbering this way.

The alternative in Word 2007 is to make use of list styles (which were
supported in Word 2003, but the advantage to using them was less apparent).
To create a list style, click Home tab | Multilevel List | Define New List
Style. Give the list style a name. To attach paragraph styles and set the
properties of the list, click the Format button, and then click Numbering.

Once created, a list style appears at Home tab | Multilevel List, under the
"List Styles" heading. You can apply it to text by clicking it or, which is
even more useful, you can right-click it and choose Modify from the context
menu; this takes you directly to the Modify Style dialog box.