I seem to have a similar problem. I had installed XAMPP a while ago, v. 1.7.3, and it worked fine, used it for over a year with no problems. Haven't used it for about a month, and now Apache does not appear to start; though, it says it has started. No green "Running" icon, "Stop" button, nor active "Admin" button (as shown below). Can start MySQL, as also shown below.
In Process Explorer, httpd.exe is not listed at all; though, mysqld.exe is listed, as well as xampp-control.exe.
There is nothing listed for this date in the \xampp\apache\logs\error.log file.
In Firefox, when I try to start a webpage or Joomla! on localhost, I get an error dialog box: "Unable to connect. Firefox can't establish a connection to the server at localhost."