Hackers utilize many different ways to infect their victims, some techniques are more successful than others. In this video below you'll see the ICQ Messenger hack I believe a lot of people will fall victim too. It sends a legitiment looking patch update message that will run the hackers backdoor when you click on it. Once your PC is backdoored with the hackers trojan, he can load auto-rooting worms, scanners, steal your personal information, steal your product keys, and more. This is why most hackers want to gain access to your PC. Backdoors do not mess up your PC or delete any information. They simply allow your PC to be remotely accessed by the hacker. Thats why these types of malware are labeled low risk by most virus scanners.