objectorbit: The game has had two versions. One named Alien Shooter: Vengeance and one named Alien Shooter 2: Reloaded. The latter has less music, less skills to worry about (Each weapon type had its own skill, and Armor had its own skill as well), stats were altered to an extent, a couple of survival modes added, and two levels added.
Oh, and they removed multiplayer.
...Yup.
They've also removed voice files, which should be played when someone contacts you via the radio. I remember getting them from the original CD version and putting into the "Reloaded" one. All this was done because, originally, the game was distributed on 2 CDs, because Internet connection at home wasn't something that most people had here in Russia (not to say, broadband internet connection with unlimited traffic), and CDs were still the main way of software distribution here, while, later, more and more people started getting the ability to get home internet connection, and became able to download software from the Internet, but data transfer speeds were still far from great, and the amount or monthly available Internet traffic was quite limited, so guys at Sigma decided that they need to make their game as compact as possible, so it could be distributed via online downloads better, so they've removed or simplified everything they thought was not necessary, and made the game simplier (most people saw Alien Shooter games as a small, quick, kinda, stress-relief alien grinder to quickly and mindlessly kill some free time, and got kinda confused when they've got almost and RPG with almost Fallout-like, complex character development system as a sequel, and not everyone liked that, so guys at Sigma decided to simplify that too, and, I guess, Multiplayer really was buggy and not really popular, so they've decided to get rid of that part of the game also.)