All that you need to do is to configure the IApplicationContext instance for your service in the same manner as you would for any other application that is configured from its App.Config file (e.g., a console app, etc.).
Add the following section handler to your App.Config file's <configSections> node:
This tells the context that its config is to be found in a node named <spring> in the App.Config file. Then add this <spring> node under the <configuration> node as follows:
<section name="context" type="Spring.Context.Support.ContextHandler, Spring.Core"/>
Once this is done, you should be able to return the context from your call to...
<!-- your objects, resources etc. here; examples: -->
...as expected in your service's Main() method.
IApplicationContext ctx = ContextRegistry.GetContext();
Let me know if this fails to work for you.