You may need to completely uninstall Sygate first, including registry entries, to get ZA working properly...
If W2K, make sure before doing a clean install of ZA, you uninstall all traces of Sygate including any virtual drivers in the TCP/IP stack and removing registry entries.
[Please backup your registry before accessing it.]
Completely uninstall ZA.
Also, if W2K, goto Control Panel - Add/Remove Hardware, tick Uninstall/Unplug - Uninstall - and tick the "Show Hidden Devices" in the 'Installed Devices" list box.
Remove "vsdatant" and "TrueVector Internet Monitor" if they are still present. *Check for any leftover Sygate stuff, too, and remove.
Reinstall fresh ZA. Then, if you still need Sygate for networking, reinstall Sygate.
This is what I would try.. hth and gl
*Edit, added.