How do I change a Windows Service's startup type in .NET (post-install)?

I wrote a blog post on how to do this using P/Invoke. Using the ServiceHelper class from my post you can do the following to change the Start Mode.

var svc = new ServiceController("ServiceNameGoesHere");  
ServiceHelper.ChangeStartMode(svc, ServiceStartMode.Automatic); 

In the service installer you have to say

[RunInstaller(true)]
public class ProjectInstaller : System.Configuration.Install.Installer 
{
    public ProjectInstaller()
    {
        ...
        this.serviceInstaller1.StartType = System.ServiceProcess.ServiceStartMode.Automatic;
    }
}

You could also ask the user during installation and then set this value. Or just set this property in the visual studio designer.

You can use the OpenService() and ChangeServiceConfig() native Win32 APIs for that purpose. I believe that there is some information on pinvoke.net and of course on MSDN. You might want to check out the P/Invoke Interopt Assistant.