not entirly linked to making the trojan undetectable but...

if you got into the computer initally (telnet or whatever you choose) then dropping a virus that would then disable or kill the antivirus then going about your original plan without needing to know ASM or having to make your own packer (given you dont have the programming experience or time).

But if you do have the time...take the other guys's advice of making your own creation...since its nicer to have your own creation instead of piggy backing off of someone elses work :P