Installer Program not running

I have a Macbook OS X that is about four and a half years old. One day, when I tried to open itunes, the itunes symbol came up on my dock, bounced a few times, and then disappeared. It won't open anymore. The same thing has now happened with the installer program. When I download anything and click on it to install it, the installer icon comes up on the dock, bounces a few times, and then disappears. How can I fix this?