Deleting A Column DEFAULT....

Hi,
I've the following problem. I must delete a column DEFAULT from a table,
but I must do it with a script, independently from the server where it'll
be executed.
Locally I've tried with:

ALTER TABLE [dbo].[PlanningDettaglio]
DROP CONSTRAINT [DF__PlanningD__OreSt__688C6DAC]
GO

ALTER TABLE [dbo].[PlanningDettaglio]
ALTER COLUMN [OreStraordinario] varchar(5)
GO

ALTER TABLE [dbo].[PlanningDettaglio]
ADD DEFAULT ('00.00') FOR [OreStraordinario]
GO


it works, but only locally.
I've tried with:

ALTER TABLE PlanningDettaglio ALTER COLUMN OreStraordinario DROP DEFAULT

Err.: Incorrect syntax near the keyword 'DEFAULT'.


Can someone help me please?

Thanks in advance,
Giacomo

P.S.
We're using SQL Sever 2000 (version 8.00.194)