The following documentation shows the process of installing and authorizing ArcGIS Desktop software. Authorization instructions start at step 11. Download the software here. Also download the .prvc file for authorizing the software here (This is a plain text file. You can download it either by right-clicking on the link above and choosing "Save Link As", or simply click on the link, so that its plain text contents are visible, and then choose "Save" or "Save As" from your browser's File menu).
Click Run ESRI.exe
Note: If you are not sure what an ISO file is or how to use an ISO file, click here.