Hi Tim, I think that something strange can be happening during installation, maybe a virus.
from FAQ 3.3
Important: Under Windows, please uninstall a previous version, if you have one installed, before installing the latest package. For sanity, also reboot your machine after uninstalling the old version and before installing the new one. You must select the Uninstall option from the Start menu, under the PixInsight Platform item, or tell Windows to uninstall PixInsight. Never try to uninstall PixInsight by manually removing files.
from FAQ 3.4
Reboot your machine before installing a new version of PixInsight. Also check your computer thoroughly for viruses, Trojans, spyware and other malwares with a good, up-to-date virus protection software. This should not happen on a clean Windows installation.
Saludos, Alejandro.