You probably tried to launch your server twice. Go to your task manager by doing ctrl+alt+delete and clicking task manager. Then go to processes. Here shut down all tasks with 'java' or 'javaw' and 'Minecraft' in the description. Now try to launch your server again. If this fails make sure nobody in your home runs a server. If that fails and you sure the server isn't launched try to define another port in your configuration from the server.