I don't see the problem on the side of xampp.
Port 80 is the standard port for http-www Server. Skype (and some other programs) aren't webserver in classical meaning, so they shouldn't use this port (same as port 443 which is the standard port for http protocol over TLS/SSL).
These programs only use these ports, because it's very common that they are not blocked by any security system.
Of cause on some systems there is already a webserver installed (e.g. IIS on some Windows Versions). Users which want to install xampp on there system rarely use this other webserver (XAMPP as testsystem for IIS doesn't make sense, more than one webserver on one system isn't recommend). So this other webserver can be uninstalled. Thats the clean way.