Hi,
Try the following, click "Start" ---> "Control Panel" ---> "Program and Features" and uninstall "PixInsight"
Now, see if there are any program files left-over, so using "Windows Explorer" find "Program Files (x86)" or "Program Files" and deleted any PixInsight files and folders.
Perhaps reboot the PC and then try install PixInsight again.
If that does not work, perhaps post a screen-shot of the error might help a little more.
Good luck!
John