I was a little surprised that MongoDB doesn't install itself as a windows service when you run the installation package. So here's a command to install it as a service. It will be set to startup type Automatic.
mongod --dbpath "C:\mongo_data" --logpath "C:\mongo_log\log.txt" --install
The dbpath and logpath are up to you. Run this as an administrator. You will have to manually start the service after you run the command. Go Mongo!