Mission Planner does not install PX4 on my laptop; six tries! Checked its (non-)existence in Windows7 Device Manager.
I am running Windows 8/8.1 on my HP laptop and the MP installer did install the necessary USB drivers on my laptop.
Did the MP installer open a window that asked for permission to install certain USB drivers? It should have. Maybe they were behind the Installation Screen, but you could not have continued with the installation without accepting the USB driver installation request.
Yes, and the installer said that 4 drivers were ready to use. BUT these do not show in Device Manager, and firmware load fails.
When you have the Pixhawk plugged into the laptop USB port have you looked at the COMM ports in the Device Manager to see if a new COMM port appears when the Pixhawk is plugged into the USB port?