Hierachical Table (count) - MS SQL 2000

Hello

for MS SQL 2000 i am having :

CREATE TABLE [dbo].[Items](
[id_Items] [int] IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
[id_ItemsSup] [int] NULL,
[Name] [nvarchar] (100) NOT NULL,
[SubItems][int] DEFAULT (0)
) ON [PRIMARY]


with :
UPDATE [Items] SET SubItems = (SELECT COUNT(id_Items) AS ct FROM dbo.Items WHERE id_ItemsSup = 1) WHERE id_Items = 1
I get how many subItems has Item = 1

how can I update the Column SubItems (for each row) ?
to get the total of subItems for each Item ?

thank you