This problem has completely shot my entire day. I reformatted my old windows XP comp and tried installing XAMPP only to find that when i try to start Apache xampp claims por
Run this in command prompt:
netstat -ano | find ":80"
It will show you what process (PID) is listening on port 80.
From there you can open task manager, make sure you have PID selected in columns view option, and find the matching PID to find what process it is.
If its svchost.exe you'll have to dig more (see tasklist /svc).
I had this happen to me recently and it wasn't any of the popular answers like Skype either, could be Adobe, Java, anything really.