A trojan, or trojan horse, is a seemingly legitimate program which secretly performs other, usually malicious, functions. It is usually user-initiated and does not replicate.
Disinfection & Removal
Trojan:W32/Agent is a very large family of programs, most of which download and install adware or malware to the victim's machine. Agent variants may also change the configuration settings for Windows Explorer and/or for the Windows interface.
Other "Agent" Families
Trojan:W32/Agent is a subset of the "Agent" family, which groups together a wide variety of malware that do not fit into any other known families. The Agent family includes trojans, worms, viruses, backdoors and other types of malicious programs.
For representative examples of these other subsets, you can also see:
A detection with the name, " BehavesLike:Win32.Malware" is a Heuristic Detection. It indicates that a file was identified as a possible threat due to its suspicious behavior. This detection is commonly triggered by malware that attempts function as, or to impersonate, legitimate applications.
For more information about heuristic detections, please see the Heuristic description.