The most common methods used by intruders to gain control of home computers are briefly described below. More detailed information is available by reviewing the URLs listed in the References section below.

Trojan horse programs

Back door and remote administration programs

Denial of service

Being an intermediary for another attack

Unprotected Windows shares

Mobile code (Java, JavaScript, and ActiveX)

Cross-site scripting

Email spoofing

Email-borne viruses

Hidden file extensions

Chat clients

Packet sniffing

source:- http://www.cert.org/tech_tips/home_networks.html