I would look into Web Services Enhancements (WSE). There are a few resources, like these that should help you get a start securing a web service with WSE.
An introduction to Web Service Security using WSE - Part I
http://www.codeproject.com/KB/webservices/WS-Security.aspxProtect Your Web Services Through The Extensible Policy Framework In WSE 3.0
http://msdn.microsoft.com/en-us/magazine/cc163650.aspx