Could be something like this:
WHEN Datt < '2009/03' THEN 0
WHEN Datt = '2009/03' THEN
SUM(CASE WHEN Datt <= '2009/03' THEN Amt ELSE 0 END)
OVER (PARTITION BY id)
YourTableIf Datt is a date, you should use a date/datetime type. IN the query above, I am assuming it is a string because it seemed so.