Howto
How to create a webservice in csharp http://wiki.developerforce.com/index.php/Creating_an_Outbound_Messaging_Notification_Service_with_CSharp_and_.Net_Framework_2.0
How to create a webservice in csharp http://wiki.developerforce.com/index.php/Creating_an_Outbound_Messaging_Notification_Service_with_CSharp_and_.Net_Framework_2.0