Word 2002 and 2003 have built-in capability for printing booklets with pages
half the size of your sheet of paper, with proper page numbering. Simply lay
out your document on A5 pages, in logical order (ie. page 1, pg 2, pg 3,
etc.). The number of pages must be a nultiple of 4 - if not, you must add
blank pages in the appropriate places, say inside front cover, or at the end.
Then File > Page Setup > Margins > Multiple Pages and select Book fold.
If you are using an earlier version of Word, see http://www.word.mvps.org/faqs/formatting/BookletPrinting.htm