401 error in ms crm 4.0

Hi I have faced the folowing error while trying to connect Asp.net pages to
ms Crm 4.0 . I tried to retrieve an entity records and assign values to the
attributes from Asp.net pages.
Hereby I have attached the code. plz help me to solve the error

401 error : Unauthorized.



CrmService serv = new CrmService();
CrmAuthenticationToken token = new CrmAuthenticationToken();
serv.Credentials = System.Net.CredentialCache.DefaultCredentials;

Guid conguid = new Guid("");

AllColumns cols = new AllColumns();
contact contact = (contact)serv.Retrieve(EntityName.contact.ToString(),
conguid, cols);