T-SQL (SS2K8) :: How To Calculate Cumulative Numbers

I want to show cumulative numbers, but don't know how to calculate them.

Here is an example of the source and the wanted result:

Source:

[Week] [Count]
1 15
2 5
3 6
4 10
(until 52)

Result:

[Week] [Count]
1 15
2 20
3 26
4 36

Is this possible, and how?

TestData:

USE TestDb /*SqlServer 2005*/
CREATE TABLE Test(
[Week] [int] NOT NULL,
[Count] [int] NOT NULL
)
GO
INSERT INTO Test ([Week], [Count]) VALUES (1, 15)
INSERT INTO Test ([Week], [Count]) VALUES (2, 5)
INSERT INTO Test ([Week], [Count]) VALUES (3, 6)
INSERT INTO Test ([Week], [Count]) VALUES (4, 10)