Authentication in Web Service

Authentication in Web Service I am developing an Webservice app that will have many methods, some methods choices can be executed by some users only, all can be executed by administrators, user are placed in Windows security groups on the W2k Server domain controller. If a user belongs to a certain group he can have access to a method and run whats available from that. I am now using Thread.CurrentPrincipal class to get the user account group domain but it returns an error of "The request failed with HTTP status 401: Access Denied"! Any help would be much appreciated. Thanks. Chris