The font (and size) used for Hindi is determined by the setting in the
Paragraph Style you happen to be using at the time -- most often this
will simply be the Normal style. Go to the Styles & Formatting panel
(Ctrl-Alt-Shift-S in either Word2003 or Word2007), right-click on the
name of the Paragraph Style you're using, and set the font in the
space labeled "Complex Fonts."
If your Styles setup hasn't been modified too much, other styles, such
as the Headings, are likely to be "based on" Normal, and this change
should percolate through them. Or, you can change this setting
individually in all the Paragraph Styles you use.
If you have occasion to use other languages, for instance you need a
few words of Bengali, then if Bengali isn't included in Kruti Dev 010
(and why would it be?), Word will substitute whichever is its default
for that script (probably Arial Unicode).