How to call a file from the arduino?
First of all you need to develop a windows app. You can achieve this by downloading visual studio express for windows desktop from here. Then you must create a project in your language of your preference. You will have to add a serial port component and set the properties to match your arduino serial ones. Then from the event that handles the "data available" of your serial port you can check if there is an incoming character from your arduino and create an if statement that runs the .exe file. Then you must add serial.print('c') to your code when there is a coin inserted. By doing so, you will manage to communicate between arduino and your windows app. 

More info and full guide can be found here