A trojan is just one piece of malware. There are a lot more kinds. The way you describe (Avast+Trojan detector) would mean you also need to install seperate detection for:
- backdoors
- virus
- worms
etc etc.
There is really no need for it. If you have Avast, a good firewall, and have your security properly setup it will be sufficient to protect your system(s).
Remember, the weakest link when it comes to computers and security, is ALWAYS the user.