It seems like the intro file is broken, so delete or rename this file:
изтриване / iztrivane / 删除 / Shānchú / 刪除 Shānchú / vymazat / slet / verwijderen / poistaa / retirer / αφαίρεση / afaíresi / eltávolítás / Удалить / Udalit / rimuovere / fjerne / gỡ bỏ :


\SteamApps\common\Jagged Alliance 2 Gold\Data\Intro.slf


then you can start the game with windows 10 x64


[UPDATE 2016]
Little Update:
there is a Modification (not tested yed, but it is open source and people told me, it works great):
https://bitbucket.org/gennady/ja2-stracciatella/src
JA2 Stracciatella is a mod. which is open Source!
The goal of the project was to make Jagged Alliance 2 available on a wide range of platforms, improve its stability, fix bugs and provide a stable platform for mod development. At the moment the goal mostly to fix bugs.