ALTER TABLE [dbo].[Users] WITH NOCHECK ADD 
	CONSTRAINT [IX_Users] UNIQUE  NONCLUSTERED 
	(
		[Login]
	)  ON [PRIMARY] 
GO

select sysconstraints.colid,sysconstraints.status  from sysconstraints, sysobjects 
where sysobjects.id=sysconstraints.id and sysobjects.name='Users'

возвращает

colid       status
 ----------------
 
 0 	 2593 
 0 	 3106 

select @@version

Microsoft SQL Server   2000  -  8 . 00 . 384  (Intel X86)   May  23   2001   00 : 02 : 52    Copyright (c)  1988 - 2000  Microsoft Corporation  Desktop Engine on Windows NT  5 . 0  (Build  2195 : Service Pack  2 )