I know you said you tried it on other computers, but give this a try on the computer that has WebUpdater installed.
Open WINDOWS EXPLORER
Click on Tools
Then Folder Options
Then open View tab
Put check mark under "show hidden files and folder"- (Windoes Default is unchecked)
UNCHECK "hide extensions for known file types" (Windows Default is Checked)
UNCHECK "hide protected operating system files (Recomended)" Windows Default is Checked)
DISREGARD any warning message that follows after modifying.
Reboot the computer and check to see if the Nuvi 765T is now recognized.