A personal computer can hold a lot of information about an individual. This information should not reach unauthorized people. Under normal circumstances it would remain private. But the Internet opens new ways for unscrupulous people to extract information from others' systems without them being aware of such data theft.
When a person opens an infected email (or more specifically a program is ran from within the email), a virus is transmitted which can leave your computer open to attack. This type of viral infection also happens through websites – anytime you download a program to run on your computer it could potentially install a program that monitors what you do on your computer and compromises your security.
Warez is a term that referes to pirated software. People tend to download such software because it is offered for free on the internet, whereas the software costs a lot if purchased legitimately. Commonly, spyware is attached to the software, that when run allows remote access to personal information in the computer.
Trojans are the most common malware programs (referring to programs that appear to be installing fine, but also install a hidden virus or spyware). Once the system is infected, the hacker gets to the business of pulling out important data such as bank account details, bank balances, passwords, etc., so that he can withdraw money. Usually, that is the real reason most hacking happens, although the hacker may also use other personal information like pictures of the individual, or personal correspondence. Identity theft has been reported to be on the rise in recent years.
So how does one prevent hacking? There are several anti-spyware programs available in the market, such as Norton, Kaspersky, AVG, etc. Another way is to install a firewall, which prompts some external attacks on computers on the internet. A combination of two or more anti-spyware methods would be of considerable help.
Since there is a good chance of failure in preventing security attacks, it is good to know the symptoms as well. The person may notice unusual behavior on the part of PC such as ads coming up without clicking anything, or applications close of their own accord. The Internet connection too can commonly become slower. All these are potential indicators of infection, and running an anti-virus check is warranted for preventing further loss of data or files.