Sum() rows

Hi.
How can I sum a row and show the sum for each row???
For example, in a finances table that have the total movimentation(debit/credit)
in the bank.

i.e:
CREATE TABLE TB1 (id integer primary key, value numeric);
insert into tb1 values (1,20);
insert into tb1 values (2,2);
insert into tb1 values (3,3);
insert into tb1 values (4,17);
insert into tb1 values (5,-0.5);
insert into tb1 values (6,3);

I want a query that returns:
-id- | --- value --- | --- subtot ---  1 |        20.00  |         20.00  2 |         2.00  |         22.00  3 |
3.00  |         25.00  4 |        17.00  |         42.00  5 |        -0.50  |         41.50  6 |         3.00  |
44.50
 

The subtot colum will be the "prev. subtot colum"+"value colum". :-/
I dont know how to make the "subtot" colum, I tried to use the sum() function
but it not works correctly.
Any idea???

Thanks.