Again, we'll need more info than that to help you out. To paraphrase "That's like asking us why your car won't start"
As a starting point please see this page:
http://www.easyphp.org/faq.php
Compare the messages there from what you are seeing on your side, it's most likely that you'll refer to this page more than once, since solving one problem might bring about a new one. Regardless, that should point you in the right direction. As a silver bullet, you could retry the installation with admin rights? If that doesn't work either, launch the app from CMD so you can see exactly what message is it's bringing back. If that doesn't help and you actually have the error message, let us know what it says, we'll be able to assist you from there.