Web Service - Authentication

Hi

I am trying to create a item list by adding web references,the following steps i have done.

1)Published the item list through NAV.
2)Created a new project through "Visual Web Developer 2008"
3)Added the "Web reference" with the published URL.
4)Placed an ObjectDataSource(speicifed the read) in ASP page.
5)Placed a "GridView" and shown the item fields by using configured ObjectDataSource.

If i try to run the program it says the following error
"The request failed with HTTP status 401: Unauthorized"
on the line "Dim results() As Object = Me.Invoke("Read", New Object() {No})

Can anyone suggest me to resolve this?
Thanks in advance.

Regards

Valan