Does it need to be a service? You can just have an executable on disk run as a scheduled task. Just set it to run as the SYSTEM account. If it needs to be running constantly, uncheck the option to disable the task after 3 days.
If you want a service look at using srvany to install your program as a service. Then just set it to automatic.
Place your executable on disk somewhere. Open up the task scheduler from admin tools in control panel. Create a basic task. give it a name of choice. Trigger is when the computer starts. action is start a program, browse to your executable.
After finishing the task, right click it open properties.
In the general tab click change user or group.
Enter SYSTEM. hit check names, it should then be underscored.
If you need the task to continuouly run for days, click the settings tab. Uncheck the stop the task if it runs longer than 3 days box.