Impersonation failed for Webservice

Hi,
We're developing a web site using asp.net 2.0 that make asynchronous calls Web service applications that we developed. Code is shown as follows:

Dim ws As New service.SamplingService
ws.Credentials = System.Net.CredentialCache.DefaultCredentials
Dim asyncWS As IAsyncResult = ws.BeginMain(job_name & ";" & job_id, Nothing, Nothing)

Both our web.config files for web site and web service have the following in web.config
<identity impersonate="true"/>
<authentication mode="Windows"/>

Everything worked fine until we started deploying the web site and web services on our client's machine. (Mind you that our development environment is 32-bit Win 2003, while client's machine is 64-bit Win 2003). Now we get the error message from server( System.Web.SOAP.Exception: Server was unable to process request. System.Data.SqlClient.SqlException: Login failed for user 'NT AUTHORITY\NETWORK SERVICE'. Does anyone know what the problem could be?

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