AVAST has improved ALOT since a few months in trojan detection. They use (they added) a very strong generic method to detect them.
This is why most of the trojans detected by AVAST doesn't have a "real name", it is just detected as a generic worm or trojan. This is not a major problem, as you can easely look up the real name when you scan the file or your pc with a online-scanner.
The problem is with trojans, that they can easely "hide" from scanners. You can use an exotic packer or hex-edit them, or add a few bytes etc...you can even download trojans that were specially made to evade detection from scanners...not to mention polymorhpic droppers.
so dealing with trojans, rootkits, backdoors is NO easy task.
Waldo