I could not replicate your problem. I tried the following:

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[Table1]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)

drop table [dbo].[Table1]

GO

CREATE TABLE [dbo].[Table1] (

[pk] uniqueidentifier ROWGUIDCOL NOT NULL,

[ln_no] [int] NULL

) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Table1] WITH NOCHECK ADD

CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED

(

[pk]

) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Table1] ADD

CONSTRAINT [DF_Table1_pk] DEFAULT (newid()) FOR [pk]

GO

INSERT INTO Table1 (ln_no) VALUES (1)

--Viewing the record in Enterprise Manager -> Tables -> Open Table -> All Rows:

--the uniqueidentifier column is stored as

{2F240FEA-0116-4915-9382-CFA4B97B9297} --NOTE curly braces {} !

--whereas the query analyzer displays

2F240FEA-0116-4915-9382-CFA4B97B9297 -- NO braces

--1. From Query Analyzer, BOTH statements, with or without braces, WORK

INSERT INTO Table1 (ln_no) values (1)

SELECT * FROM Table1 WHERE pk = '{2F240FEA-0116-4915-9382-CFA4B97B9297}'

SELECT * FROM Table1 WHERE pk = '2F240FEA-0116-4915-9382-CFA4B97B9297'

pk ln_no

2F240FEA-0116-4915-9382-CFA4B97B9297 1

--2. Checking off IsRowGuid produces the same results

INSERT INTO Table1 (ln_no) values (2)

SELECT * FROM Table1 WHERE pk = '{707A6054-A363-4263-8CB7-F0776A518DA8}'

SELECT * FROM Table1 WHERE pk = '707A6054-A363-4263-8CB7-F0776A518DA8'

pk ln_no

707A6054-A363-4263-8CB7-F0776A518DA8 2

--Removing the default NEWID() property and entering it explicitly STILL WORKS

INSERT INTO Table1 (pk, ln_no) Select NEWID(), 3

SELECT * FROM Table1 WHERE pk = '{DE269F48-45DC-4BBB-8FD7-9A474E2906D6}'

SELECT * FROM Table1 WHERE pk = 'DE269F48-45DC-4BBB-8FD7-9A474E2906D6'

pk ln_no

DE269F48-45DC-4BBB-8FD7-9A474E2906D6 3

Tp help ypu further, you have to suly more info. Script your table and insert values.