EXAMPLE :
WEEK TOTAL T
----------- ----------- -----------
3 5 14
2 6 8
1 5 3
0 3 0

SELECT T2.WEEK, T2.TOTAL, COALESCE(SUM(T1.TOTAL), 0) AS T
FROM MY_TABLE T1
       LEFT OUTER JOIN MY_TABLE T2
             ON T1.WEEK < T2.WEEK
GROUP BY T2.WEEK, T2.TOTAL
ORDER BY 1 DESC

Here I use the COALESCE function that will replace the NULL that can be found in some rows by 0 (otherwise the sum couldn't be done)

Nearly the same but last column is current total + previous like :
week total t
1 5 5
2 6 11
3 5 16
SELECT t2.week, t2.total, sum(t1.total)
FROM  MY_TABLE T1,  MY_TABLE T2
 WHERE T1.WEEK <=t2.week
GROUP BY  t2.week, T2.total
ORDER BY t2.week ASC

The following feature outside Core SQL-99 is used:

T611, "Elementary OLAP operations"
(checked with the mimer sql validator)
SELECT week
     , total
     , sum(total) over (ORDER BY week DESC) AS running_total
FROM my_table