Quote from the instructions on the software distribution site:
"Windows: We provide standard Windows installation packages. To install one of these packages, simply launch the executable file and follow the instructions. Once installed, you'll have a new entry on your Start > All Programs menu, namely PixInsight Platform, where you'll find an option to launch the PixInsight Core application. The installation also includes an uninstall utility.
Important: Under Windows, please uninstall a previous version, if you have one installed, before installing the latest package. You must select the Uninstall option from the Start menu, under the PixInsight Platform item. Never try to uninstall PixInsight by manually removing files. "