Source : http://oolacola.blogspot.com/2009/04/how-to-find-out-which-application-is.html
Let's say that we are looking for port 80 -- IIS, Apache and other web servers listen in port 80, so when you are having problems starting Apache, this technique will be useful. Here is the command.
C:\>netstat -aon | findstr 80
-a means list all active connections and their ports. -o means include their process IDs. -n means display the port numbers numerically.
The pipe symbol ( | ) means, that instead of the result of netstat being displayed on the screen, feed it's result to the findstr process -- we are looking specifically for the line which has 0.0:80 -- you actually don't need findstr, but I don't want to scroll down and hunt down manually which app is using port 80. You might see something like this.
TCP 0.0.0.0:80 0.0.0.0:0 LISTENING 560
Aha! now we know that process 560 is using port 80 (that last column right there, is the process ID), you could press CTRL-ALT-DEL now to show the process window, and manually look up which app has the process ID 560, or you could enter ..
C:\>tasklist | findstr 560