Howto make this Windows-Service-Scenario as comely as possible

Answer 1

I would definitely use WCF as the communication layer between the web-app and the service. You can host a ServiceHost in your windows service, and serve up any type of WCF endpoint.

A common pattern I've seen is to connect a web layer and service layer using MSMQ (Net MSMQ binding), so that you have disconnected calls, and some buffering to allow for load tolerance. If you don't need the buffering, you can use any other type of binding (Net TCP or even HTTP, although sometimes it tricky to get HTTP setup correctly outside of IIS).

Here's a good tutorial:

http://msdn.microsoft.com/en-us/library/ms733069.aspx