Since the OP asked about a "numbered list," I took "decimal point" to mean
the periods following the numbers. It's really only possible to align Level
1 numbering this way satisfactorily, by using Right alignment. Beyond that,
with combinations of one and two digits at two or more levels, you can't
align *all* the periods, though you can align on the last one.
I seem to have managed to get my default single-level numbering
right-aligned in Word 2007, as a result of playing around trying to figure
out how to update
http://sbarnhill.mvps.org/WordFAQs/NumberAlignment.htm for
that version (I've concluded that the procedure is so different that I'm
going to need a whole new article for Word 2007).