set anonymous access in your virtual directory
write following credentials to your service
ADTService.ServiceClient adtService = new ADTService.ServiceClient();
adtService.ClientCredentials.Windows.ClientCredential.UserName="windowsuseraccountname";
adtService.ClientCredentials.Windows.ClientCredential.Password="windowsuseraccountpassword";
adtService.ClientCredentials.Windows.ClientCredential.Domain="windowspcname";
after that you call your webservice methods.