Couldn't you simply do this?

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[isuniqueidentifier]') and xtype in (N'FN', N'IF', N'TF'))

drop function [dbo].[isuniqueidentifier]

GO

CREATE FUNCTION dbo.isuniqueidentifier (@unique varchar(100)) 

RETURNS bit AS 

BEGIN...