Depending on the version of Windows in use and the Windows options that have been installed, there are multiple ways to host .NET programs in the background. Chapter 11 covered Web Services and Windows Communication Foundation (WCF) services, both of which are examples of technologies that can use Internet Information Services (IIS) to load programs and run them independent of the user.
If you are using IIS 7.0 or above, you also have the option to run WCF services using the Windows Process Activation Service (normally called WAS). This allows hosting of WCF services using non-HTTP protocols such as TCP. As with IIS, no code needs to be written; only the proper configuration is necessary.
Another option that offers more control over when and how background programs are loaded is called Windows Services. The basic concept goes back to Windows NT, when this capability was called NT Services.