If you are running the Development Application Server, changing the port used by the server is the easiest solution. Change the Server Port in the Application Server Control Panel and start the server. Specify a port that is not 80, such as 8080.
Changing the port number may not be desired in a Production environment. This is because anyone using your site must include the port number in the URL.
You have to follow few instructions below to determine the application using the port:
1. At a Windows command prompt, run the command netstat -o. This will output a list of ports, along with the PID (process ID) that has that port open.
2. The sample output above indicates that the local port 80 is being used by process ID 2636. To determine the executable running as this process ID, you need to use Windows Task Manager.
Open Windows Task Manager. This can be done in the Windows command prompt. Run taskmgr in the command prompt.
C:\Documents and Settings\Administrator>taskmgr
C:\Documents and Settings\Administrator>
3. Open the Processes tab.
4. Add PID (Process Identifier) to the columns displayed for the processes. Depending on the version of the Windows Task Manager you are running.
5. If Select Columns is not listed under the View menu, right-click on the title bar in the Process tab. You can refer to this for an in-depth view https://documentation.alphasoftware.com/documentation/pages/Troubleshooting/Web%20Server/Port%20is%20in%20use.xml