Add/remove columns, changing default values in a SQL Server 2005 table

Add new column in a table with checking for existence.
IF NOT EXISTS(SELECT * FROM syscolumns WHERE id=object_id('ColumnName') and name='TableName')
 ALTER TABLE TableName ADD ColumnName bit not null default 0

Changing default values in a SQL Server 2005 table
ALTER TABLE [Table]  ADD CONSTRAINT
    ConstrName  DEFAULT 0 FOR ColumnName