I don't have that file in my zone alarm installation, nor, after a search, anywhere on my hard drive.
It appears to be a toolbar for ZA according to google results
The following is the available information on tbZone.dll:
Property Value
Product name Conduit Toolbar
Company name Conduit Ltd.
File description Conduit Toolbar
Internal name Conduit Toolbar
Comments Conduit Toolbar ver 1.0
Legal copyright Copyright © Conduit Ltd. 2008
Legal trademark Copyright © Conduit Ltd. 2008
Private build 4