Skype interferes with XAMPP on port 80

When Installing XAMPP it could not start the Apache HTTP server. It appeared to be a problem with using port 80.

In the xammp or xampplite directory, there is a utility named "xampp-portcheck.exe" - running it will tell you if port 80 is "free", or if it is not it may show "SYSTEM" for example. If this test does not show port 80 is "free" proceed with the settings below.

When installing wampserver the port 80 test determined that Skype might be using it.

After closing Skype and the server was able to run and WAMP came online displaying the test pages - working fine.

In Skype options: [connections], there is a checkbox labeled "Use port 80 and 443 as alternatives for incoming connections".
This is our problem, so simply uncheck it and save. It may require a Skype restart.

You should now be able to start the WAMP, XAMPP or XAMPPlite server again. If you still have trouble, try closing Skype.
It's also possible to configure Apache to use other ports if necessary. See Apache docs at http://httpd.apache.org/docs/2.2/bind.html#comment_588

There are some other net applications (like P2P apps, chat applications, Media Centres or, of course, other web servers like IIS) that also sometimes grab port 80. It's a common method to get through firewalls when the normal protocol is blocked.

One way to test if 80 is in use
Ensure the expected web server is not running.

At a command shell:
telnet localhost 80

What we want to see immediately is "Could not open connection to the host, on port 80" and a return to the command line. This means that port 80 is not in use already, which means the web server can now be started.

If you see anything else - especially a PURE BLANK SCREEN for more than a few seconds, it means that something is listening ... and occupying that port. Depending on the application it may be hard to quit, so press Enter a few times, or just close the DOS box.
Now go and start shutting down net-type applications and services you may be running.

Another way to test if 80

Run in the command shell:
netstat -o -n -a | findstr 0.0.0.0:80

If output will be something like this:

  TCP    0.0.0.0:80             0.0.0.0:0              LISTENING       1776

In last column, there is process id (pid).

To find what application is it, start Task manager. If you don't have a column with PID, go to 'View / Select Columns' and check 'PID (Process Identifier)'. And now you can look what process run is it.

Documentation on changing common port numbers

The common ports constitutes of Apache port, by default which is 80, SSL port 443, and the biggest toubled port number is 3306. To change the common port number you find many handy online guides. Swapreference guide on How to change Common MySQL port in Xampp can be checked. 

Before changing port number read tutorial/guide carefully, and take backup to revoke any changes, which can cause later issues in local development environment. 

To make your development process smoother, you can use ddev virtual system deployed in Docker.