Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
This is a botnet that installs mIRC in order for the distributor to gain a remove connection to the infected machine. Once the IRC client is installed, the computer is given a Nickname based off a separate text file that is dropped. Research has concluded that this botnet originates from a crack serial site.