How can I drop all the default constraints constraints on a table

One solution from a search: (Edited for Default constraints)

SET NOCOUNT ON

DECLARE  @constraintname SYSNAME, @objectid int,
           @sqlcmd         VARCHAR(1024)

DECLARE CONSTRAINTSCURSOR CURSOR  FOR
SELECT NAME, object_id
FROM   SYS.OBJECTS
WHERE  TYPE = 'D' AND @objectid = OBJECT_ID('Mytable')

OPEN CONSTRAINTSCURSOR

FETCH NEXT FROM CONSTRAINTSCURSOR
INTO @constraintname, @objectid

WHILE (@@FETCH_STATUS = 0)
BEGIN
    SELECT @sqlcmd = 'ALTER TABLE ' + OBJECT_NAME(@objectid) + ' DROP CONSTRAINT ' + @constraintname
    EXEC( @sqlcmd)
    FETCH NEXT FROM CONSTRAINTSCURSOR
    INTO @constraintname, @objectid
END

CLOSE CONSTRAINTSCURSOR
DEALLOCATE CONSTRAINTSCURSOR

I know this is old, but I just found it when googling. A solution that works for me in SQL 2008 (not sure about 2005) without resorting to cursors is below :

declare @sql nvarchar(max)

set @sql = ''

select @sql = @sql + 'alter table YourTable drop constraint ' + name  + ';'
from sys.default_constraints 
where parent_object_id = object_id('YourTable')
AND type = 'D'

exec sp_executesql @sql