<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.web>
    <httpHandlers>
      <add verb="*" path="*.xml" type="FooBar.XmlHandler, FooBar" />
    </httpHandlers>
  </system.web>
</configuration>

namespace FooBar
{
    public class XmlHandler : IHttpHandler
    {
        public bool IsReusable
        {
            get { return false; }
        }

        public void ProcessRequest(HttpContext context)
        {
            HttpResponse Response = context.Response;
            Response.Write(xmlString);
        }
    }
}