NIS Update vs Upgrade

On 2 of my PCs I have been seeing popup windows with the message "Norton Internet Security cannot be installed because you have another version of Norton Internet Security installed. Remove the existing version, restart your computer sand run the installer again."

Well, I never ran the installer in the first place, so what caused it to run? And, if I do uninstall the current version, where can I find the installer program for the new version?

On a third PC, the new version was installed automatically without any of this happening. I don't understand why this is not the case for the other 2 machines.

Can someone please explain to me what is happening here?

 

Thanks,

KMetz