port 80 problem during installation

On Linux or another Unix system you could find this out pretty easily. However, I don`t know how to do it with Windows.

That said, port 80 is normally used for HTTP--a web server. Only one program can be bound to the port at a time and the program for ports 1 to 1023 must be running as the admin or root user in a properly-designed system. When you try to run Apache, it is basically saying: "I can`t do that, Dave, someone is already using the port." If it was really helpful, it would tell you what program was using it. Most likely you have turned on Microsoft`s IIS webserver at some point and it is still running. If you can see about turning it off, port 80 will be available for Apache.

You can run Apache on another port. One popular choice is 8080. However, this means that the URLs have to note this nonstandard port like this:
http://localhost:8080/directory/script.php

if you were using your local web browser to connect to your local webserver on port 8080 to find a file called script.php in the web space (DocumentRoot in Apache) in a directory called "directory".