The error occurred during an upgrade install on a particular (single) machine. However we were able to eliminate the error message after a reboot.
We've never had to reboot before during any kind of install, upgrade or otherwise. So maybe it was something unique to that particlar machine, or something to do with Vista UAC? Either way, I wish that the error message was something more "user-friendly".
Since our installer calls the VSP driver installer, which generates the error, I don't think there is much we can do about it on our end, unless you have a suggestion. I would like to anticipate these issues before we make a release and make our install/upgrade process as seamless as possible.