This means you are trying to run a 64-bit version of Ubuntu while your computer only supports the 32-bit version. You should get the 32-bit Intel / x86 / i686 / i386 version (several terms for the same thing…).
Since the version to download is chosen by the architecture of the computer where you are downloading it, you may rather want to choose the image manually at http://releases.ubuntu.com/.
As JonasCz mentioned already, you should consider choosing a lighter edition of *ubuntu. For example, Xubuntu is my favorite. There are more, like Ubuntu MATE, Ubuntu GNOME or Lubuntu which is considered to be the lightest one.