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The installer has found the previous installation's Uninstall registry key and is attempting to run the uninstaller. This fails, because you deleted the uninstaller, so the installation aborts. We can add some robustness for this situation, but your solution will have to be to further manually uninstall Npcap by editing the Registry (be careful, you may irreparably damage your system if you make other changes):
At this point, you should be able to run the Npcap installer, which ought to safely overwrite any remaining installation artifacts. I'm not entirely sure this will work for the npcap driver and service, though.
For future reference, when you encounter a problem with the installer, manually removing parts of the install will make it much harder for us to diagnose and help you.