Rachael, make sure you have an up to date anti virus...try to have an up to date spy ware program, if you have a firewall even better or a router is fine as well...just make sure you check like at least once a week for updates & check your computer one a week for spy ware & viruses I think you should be fine if you do those things!
Hackers are hackers, they will always look for a back door to any & all programs every program has a front & back door that is how programs are made...cant help anything about that.
Most of the time trojans infect computers by asking you if you would like to install something, having a firewall will change nothing. Having an up to date virus scanner might stop it from installing but in the first place you should be careful what you say yes to online.
How do hackers hack computers? lol I guess they have ways of bending the rules just to their taste so their victim's computer goes down and they can hack into their personal information stored sort of taking on a new identiy