I am sure you might be knowing about what is a trojan however let me give a little information about the same.
Trojan horse is a file which comes as a gift & then it open doors for other virus to come in..
Now the question "how do we research what the threat is to see what possible damage it can do?" As per the technical writeup
http://www.symantec.com/security_response/writeup.jsp?docid=2004-021914-2822-99&tabid=2
(Symantec antivirus programs use Trojan horse as a generic detection when detecting many individual but varied Trojan horse programs for which specific definitions have not been created.
In these cases, a generic detection is used because it protects against many Trojans that share similar characteristics.
If a malicious program does not infect other files and does not automatically distribute itself, the program is usually labeled a Trojan horse.)
I hope this answer the above question....
But in situations where "Symantec detects a trojan horse and is unable to succesfully take care of it" .... Then it there could multipal reason for why Symantec is detecting it & not taking any action on it... This will depending upon the situations....
I would suggest you submit the file to our security response.