Mongodb as windows service
I would  recommend a Configuration File instead a bunch of command line options.

There have a closer look at Windows Service Options

Once you created the configuration file you can install the service like this:

mongod.exe --config c:\MongoDB\config\mongo.cfg --install


Note, the default service name is MongoDB. Unless you specify processManagement.windowsService.serviceName you have to use os.system('net start MongoDB') in order to start the service. 

However, if you like to run more than one MongoDB service, the service names have to be distinct.

Running start mongod ... starts the service in foreground and the terminal remains open.