Trojans like any other client/server based technology allow a program on a remote computer to "serve" its connection to a client that interacts with it, allowing almost total control over a computer without being present at the console, of course depending on functionality...
Trojans are commonly refered to as "virus's" and are technically malware because they allow unauthorized access, some virusand spyware scanners pick some up, some dont, its almost a shot in the dark most of the time.
as far as i know a trojan server (the file you send to a target) can be bound onto any program preferebly a .exe or .jpg but functionality of the original file may or may not be possible, say you bind your trojan server onto a keylogger.exe file well when the target opens the .exe its likely he wont open the keylogger but start the server on his machine. Thats why i said it may be easier for you to just install it via a trojan because you pretty much have control over a machine.
there are so many different variables of trojans available on the net, id say its a good idea to try different ones because you never know when a scan program will detect your server.
If i were me
i would set up a 2 computer network and test different trojans wih a variant of different scanners (virus/spyware etc...) and see what works best before attempting on the www
and always remember tampering with computers that are outside your ownership or permissions is illegal