As I recall, 2005 doesn't support windowed functions, so you have to deal with any running totals problem (decrementing from a total falls under the category of running totals) in one of two ways: Either use a loop or a cursor to do row by row decrements, or use the infamous quirky update method.
Read this to get you started:
http://www.sqlservercentral.com/articles/T-SQL/68467/[/url]
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