The Problem is When you try to assign @isUser = 0 , A uniqueidentifier cannot be set to Zero, But you can do something like this....
CREATE PROCEDURE [dbo].[UCP_ValidateUser]
@UserName nvarchar(32),
@isUser UNIQUEIDENTIFIER Output
AS
Begin
SET NOCOUNT ON;
SELECT TOP 1 @isUser = UserID
From UCP_Users
Where UserName=@UserName
if (@@RowCount <> 1)
BEGIN
SET @isUser = NULL
END
End
GO
And once you have assigned NULL Value to the @isUser variable then you can do further checks and populate a variable or return a value. doing something like....
IF ( @isUser IS NULL)
BEGIN
/* Do something here */
END
ELSE
BEGIN
/* Else do something */
END