If you're interested in downloading the Unreal Tournament 2004 software, which will allow you to play the game online, you can find it available for free from a number of online gaming websites. The game itself comes with some basic levels and a few bonus packs containing items that will help you get started. The game's biggest draw, however, is its single-player campaign, which is highly competitive and offers a challenge to the players' skills. While it does not have the single player component of CounterStrike or DayZ, it does have a great deal more depth, and that combined with the ability to connect with a network of players and forge alliances against those who pose a threat to your station are the main selling points of this online shooter.
If you'd prefer to download the game, the best way to do so is to use a tool called a "rogue antivirus" program to remove any potential viruses that may be lurking on the CD key or inside the game files. You can then install the game and, if you've enabled steam play, use the included CD key to activate the game using your computer's CD drive. It's best to try it out before buying the product - many people find that the single player campaign is very challenging, and you'll probably be hooked once you get started.