To call a Web service using basic authentication us the following;
using System.Net;
localhost.MyWebService myService = new localhost.MyWebService();
System.Net.CredentialCache myCredentials = new System.Net.CredentialCache();
NetworkCredential netCred = new NetworkCredential("UserName", "Password");
myCredentials.Add(new Uri(myService.Url), "Basic", netCred);
myService.Credentials = myCredentials;