Normally, it's just too easy to install xampp. Normally
There's no step-by-step way to find that program. YOu have to search for your own.
As I said, often it's just the IIS still running as a service. Normally it's not the firewall, because you can't get the apache running on the machine itself and the firewall only blocks the traffic between networks.
Please go through all running services at look for any app which could act as a webserver like the IIS. Stop that service and try to start apache again.