using trojan refer to as crakers or scriptkiddies,lamer, etc.. which uses downloadable software in the net that may aid them... A real hacker doesn't not use much of a trojan but uses their knowledge to bypass the computer, they think like computer and go inside the computer with the use of computer vulnerabilities... i had a conversation with a person who uses to be a system admin... he told me that computer has a lot of holes or backdoor he only find those holes and uses it to break into computer rather than using trojan , but if he uses trojan he asure that he made it bye himself , as far as i know every computer that is connected to internet even if it is on WAn.LAN,MAN etc.. can be a suspectible victim of an attacker..

Using firewall will protect you from those attacker only if it is properly convigured, a firewall defenes is based on the knowledge of the user, if you use a well known firewall for example but dont know how to configured it on your system the firewall may help the attacker to point your weaknesses in the system...

there is no machine that is connected to the internet can assure of 100% but they can only assure the some privacy of there system..