The use of ArcObjects within a .NET Windows Service is not currently supported under any ESRI product or developer license.
However, ArcGIS Server provides both a framework and a licensing model to use ArcObjects within a SOAP web service. The ArcGIS Server model can provide the desired ArcObjects-based GIS functionality to a small, focused group, or to a large audience. ArcGIS Server application web services can be created with either Java or .NET, and can be deployed on a large number of platforms.
For more information, see the link for the ArcGIS Server Product Site in the Related Information section below.