Vejam como resolver esse erro que aparece quando tentamos iniciar o APACHE.

Erro:
Problem detected!
Port 80 in use by “Unable to open process” with PID 4!
Apache WILL NOT start without the configured ports free!

You need to uninstall/disable/reconfigure the blocking application
or reconfigure Apache and the Control Panel to listen on a different port
Attempting to start Apache app…
Status change detected: running
Status change detected: stopped
Error: Apache shutdown unexpectedly.
This may be due to a blocked port, missing dependencies,
improper privileges, a crash, or a shutdown by another method.
Press the Logs button to view error logs and check
the Windows Event Viewer for more clues
If you need more help, copy and post this
entire log window on the forums
Problem detected!
Port 80 in use by “Unable to open process” with PID 4!
Apache WILL NOT start without the configured ports free!
You need to uninstall/disable/reconfigure the blocking application
or reconfigure Apache and the Control Panel to listen on a different port