I want to write a web service using Visual Studio. The service needs to support some type of authentication, and should be able to receive commands via simple HTTP GET requests. The input would only be a method call with some parameters, and the responses will be simple status/error codes. My instinct would be to go with an ASP.NET Web Service, but this isn't an option in C# 4.0 and it makes me wonder if I should be using something that's more up-to-date. I've looked into WCF, but it seems like this requires a running application on the client-side - is there a way to query a WCF host by just accessing a URL?

The authentication is also an important piece. Developing my own little authentication system seems like a bad idea - I've read that it's too easy to mess up. What would be the standard way of authenticating with a web service like this?

I'd love to look up all of the specifics on this and learn it myself, but I really don't even know where to begin. Some direction would be greatly appreciated!