Works everytime, consistently.

If you have not installed SP1 for Vista, delete HZ from your system, install Vista SP1 and make sure your system is fully patched, including the latest driver updates.

Re-Install Horizons.

Go to control panel, system and maintenance, then system. On the left, click on advanced system settings.
Under the performance area, click on settings. Click on the data execution protection tab. Select the radio button that says enable DEP only for essential windows programs and services

After this is complete, reboot computer.

Now, with this method you cannot use the launcher..it does not work. The game must be started from the browser at play.istaria.com. As such this site must be added to trusted sites.

In explorer, on the bottom of the screen is a little globe. Next to it, it says Internet protected mode and it will be on or off. For the game to launch from the browser it must be set to off. Take whatever action you must to turn off this protection.

Now start the game as you would normally from the browser. Under the launch option make sure that all the option are UNCHECKED. Full scan etc, .net patcher, etc...ALL must be unchecked

Then click on play.

This is tested and works repeatably for me.