Windows firewall is pretty decent firewall. The only downside is, that it's only one way firewall (protects inbound traffic, but not outbound).
Then, a firewall has nothing to do with protecting a computer against trojans, or any other type of malwares. The main firewall job is to make your computer invisible to the outside world, and block any un-authorized communication between the computer, and the outside world (traffic going in, or out).
Only use it if you're behind a hardware firewall.
Source, if you have one?
As for my source....I deal with security issues on couple of boards, and I know, many people are using just Windows firewall without any router. I still can't see any relationship between using just Windows firewall, and severity of infections on such computer.
A firewall doesn't keep trojans out
As I said, it's not firewall job, BUT, if your firewall works fine, then any trojan becomes useless, since it can't communicated with the planter, or at least gives you a warning of a such attempt.
P. S. No reason to get personal. We can somply discuss facts.