Through web pages and email (the two most popular Internet service) to a computer can get into dangerous elements. From this it is necessary to be able to defend himself! The system must be installed a reliable anti-virus, which will check all the files from the network, the malicious code. Spam can also be a source of danger to a PC (unsolicited mail more often contain a 'Trojan Horses'), so be sure to install the e-mail spam filters. However, users still have to remain vigilant – often with the help of simple techniques attackers forced him to perform actions that lead to an infection (for example, go to certain link in the message, run the attached file, etc.). This may be an Internet worm (self-replicating computer program that spreads mainly through the network. Some worms get on PC through opening an infected e-mail-message attack other computers in automatic mode), and backdoor-virus (making unauthorized control of your computer.