The answer is
Right click on "Command prompt" or "PowerShell", in menu click "Run as Administrator" (on Windows XP you can just run it as usual).
As Rick Vanover mentions in See what process is using a TCP port in Windows Server 2008
The following command will show what network traffic is in use at the port level:
Netstat -a -n -o
or
Netstat -a -n -o >%USERPROFILE%\ports.txt
(to open the port and process list in a text editor, where you can search for information you want)
Then,
with the PIDs listed in the netstat output, you can follow up with the Windows Task Manager (taskmgr.exe) or run a script with a specific PID that is using a port from the previous step. You can then use the "tasklist" command with the specific PID that corresponds to a port in question.
Example:
tasklist /svc /FI "PID eq 1348"
Use NETSTAT on the command-line:
netstat util
Use this nifty freeware utility:
CurrPorts is network monitoring software that displays the list of all currently opened TCP/IP and UDP ports on your local computer.
Type in the command:
netstat -aon | findstr :80
It will show you all processes that use port 80. Notice the pid (process id) in the right column.
If you would like to free the port, go to Task Manager, sort by pid and close those processes.
-a displays all connections and listening ports.
-o displays the owning process ID associated with each connection.
-n displays addresses and port numbers in numerical form.
If you want to be really fancy, download TCPView from Sysinternals:
TCPView is a Windows program that will show you detailed listings of all TCP and UDP endpoints on your system, including the local and remote addresses and state of TCP connections. On Windows Server 2008, Vista, and XP, TCPView also reports the name of the process that owns the endpoint. TCPView provides a more informative and conveniently presented subset of the Netstat program that ships with Windows.
Source: Stackoverflow.com