You can use OUTER APPLY:
SELECT T.Month,T.Amount,T2.Total
FROM Table1 T
OUTER APPLY
( SELECT Total = SUM(Amount)
FROM Table1 T2
WHERE T2.Month <= T.Month
) T2;
Or a correlated subquery:
SELECT T.Amount,
( SELECT Amount = SUM(Amount)
FROM Table1 T2
WHERE T2.Month <= T.Month
)
FROM Table1 T