Only one process can use port 80 at a time. Port 80 is the default port for web servers, so when you navigate to websites over HTTP, you are actually navigating to that server's port 80 by default (when you use HTTPS, the port is 443).
You can try to hunt down all the programs that are running on port 80, but there's an easier way that will work for development. When running XAMPP, click "Config" under "Apache". Replace Listen 80
with Listen 8080
and ServerName localhost:80
to ServerName localhost:8080
.
Then, when you want to look at your masterpiece, navigate to http://localhost:8080
in your browser.