【正文】 barely rejects threats form internal users。 Some hackers use these viruses to disseminate their political thought and ideas, such as small ball virus and Ambulance car virus. Vicious virus would destroy data and systems, resulting in paralysis of the entire puter, such as the CHI virus, Porch virus. Once the virus attack, the consequences will be irreparable. It should be noted that dangerous are a mon feature of puter viruses. Benign viruses are not dangerous, but the risk of the consequences of relatively light. Virtuous is a relative concept. In fact, all puter viruses are malignant. Precaution measures on puter work security In order to protect work resources, we should conduct certain ladder of management and legal instruments. Besides, we must carry out relevant technological measures aimed at the problems existed in work security to acplish the protection for work and data. After that, regular circulation can be guaranteed and profits of legal users can be ensured. At present, precaution measures dealing with work security treats are as follows: Firewall technology, Encryption technology, Access control technology and Virus protection technology. Firewall Technology As a present technological measure for protecting puter work security, firewall can part local work and major work and restrict information access and transmission between protected work and exterior one. 14 Firewall is the instrument that closes off unsafe factors on work topology structure and service type to improve work. What it protect is one of the modules with definite close borderline in work, while what it keep away is threat outside the protected work. As a result, it is firstly fit for specialized work in panies, especially for the connection with public basic functions of firewall: Filtration. It can reject TCP/IP sent by unauthorized host puter, and reject accepting the link requirement by unauthorized service. Network addresses translation. Translating IP address of inter host puter to avoiding being detected by external monitor, or we can say IP disguise. Proxy Services. On behalf of host puter to make applied connection with high level, and pletely interrupt connection between inter host puter and external one in work layer. What we should pay more attention to is none of the firewalls can provide absolute protection. Firewall has boundedness, which including uselessly protects from other attacking ways outside firewall。s attempt to show. By using the knowledge that we have learned, we can know the Computer Virus Classification as follows: （ 1） by the parasitic Category By parasitic, puter viruses can be divided into leadvirus, file virus and mixed virus. Boot virus parasites in the disk guide meaning those parts of a puter virus. It is a mon 13 virus, the use of puter systems do not usually check the guide part of the content is correct weaknesses, and retained in memory and monitor system operation, one has the opportunity to infection and destruction. According to the location of parasites in the disk, it can further be divided into the Master Boot Record boot record viruses and paragraph virus. The former master boot hard drive transmission parts, such as marijuana virus, 2708 virus, porch virus。 which can attack more than 90% of microputer. The characteristics of puter viruses are: （ 1） infection: a puter virus as a program that can replicate itself to other normal procedures or systems of certain ponents, such as the disk part of the guide. This is the basic characteristic of the virus program. With the increasingly extensive work development, puter viruses can be widely disseminated through the work in a short time. （ 2） latent: hidden in the infected system the virus does not immediately attack。s a ma nmade destruction with propose that can be divided into initiative attack and passive attack. Initiative attack aims to wreck work and information, usually using ways of modification, delete, falsifications, deception, virus and logical bombs. Once succeed, it could stop operation of work system, even a paralysis of overall system. Passive attack is to get information, which is usually conducted to steal secret information that on one is aware of, such as business and 11 trade secret, project plan, bid figures and personal information, without affecting regular operation. Hostile attack, both imitative and passive, could damage badly puter work, lead to leaks of confidential data, finally cause irreparable lost. Software leak and backdoor (puting) There are two kinds of software leaks: one is made by propose, which is intently designed to control system and steal information for the future use。s just insignificant Unicode without ay importance. Integrity means work information can be kept not being mod