A hacker will surf the internet looking for a seen computer addresses . Then he or she can try and find
a open software port on your Pc to use to plant program code on your hard drive.
Often the computer owner/user is oblivious to it's existence . This program code is known as
a "Trojan horse" and are given names based on the port number they attack through ,for example backdoor subseven Trojan's use port number 54283 .
The affect of Trojan horse on a system can be any of the following :
Many Anti-virus and firewall software include protection against many different Trojan horses and viruses but you Must keep definitions and software up to date.
Even then nobody is totally safe from the determined hacker.