install apache

[kanaghadhara]

7:57:01 PM [Apache] Problem detected!
7:57:01 PM [Apache] Port 80 in use by "Unable to open process" with PID 4!
7:57:01 PM [Apache] Apache WILL NOT start without the configured ports free!
7:57:01 PM [Apache] You need to uninstall/disable/reconfigure the blocking application
7:57:01 PM [Apache] or reconfigure Apache and the Control Panel to listen on a different port
pls help me out in resolving this error