Question: How do I build a XML-RPC web service in C#?

Answer: 
You could implement either a generic handler (.ashx) or an ASP.NET 2.0-compatible service (.asmx). You would then need to handle the XML parsing and construction either using .NET classes for XML or just on your own.

Edit: I took out information about WCF since the question changed, making it irrelevant.
if you mean RPC where you cliend simply sends raw XML via form variables, etc, and you parse it in the implementation of your service, .net 3.5 and wcf support out of the box POX services.

<http://msdn.microsoft.com/en-us/library/aa395208.aspx>