Let's say that you work at a company with 500 employees. The company will therefore have hundreds of computers that all have network cards connecting them together. In addition, the company will have one or more connections to the Internet through something like T1 or T3 lines. Without a firewall in place, all of those hundreds of computers are directly accessible to anyone on the Internet. A person who knows what he or she is doing can probe those computers, try to make FTP connections to them, try to make telnet connections to them and so on. If one employee makes a mistake and leaves a security hole, hackers can get to the machine and exploit the hole.

First, you should install computer software written by professional security specialists. This software is responsible for securing your computer and data while you are working or playing on the Internet. Attacks against computers are very common on the Internet. Hacker attacks are dangerous, not only because of possible damage to your data and documents, but through leaking confidential information, (e.g. bank accounts or numbers on your credit cards) to an unauthorized individual.

During such an attack, the hacker takes the opportunity to use your computer without your knowledge, appearing on the network in your name. The hacker can use your computer as the bridgehead to attack other computers (for example, banking or military computers). During such an attack, Internet Service Provider logs will record the attack. The log information indicates that the attack originated from your computer.Finally, the hacker may not be skilled enough to successfully attack your computer. An unsuccessful attack may jeopardize the workability of your computer (malfunction) or result in the loss of data. You will need to ask a professional to reinstall Windows and all other applications, wasting your time and money.
Additionally, gaining access to your PC, the hacker uses your Internet connection which is paid for by you.