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); 

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.