【正文】 ther malicious code. Since viruses code must be executed to have any effect, files that the puter treats as pure data are safe. This includes graphics and sound files such as .gif, .jpg, .mp3, .wav, .etc., as well as plain text in .txt files. For example, just viewing picture files won?t infect your puter with a virus. The virus code has to be in a form, such as an .exe program file or a Word .doc file which the puter will actually try to execute. How do viruses spread? The methodology of virus infection was pretty straightforward when first puter viruses such as Lehigh and Jerusalem started appearing. A virus is a small piece of puter code, usually form several bytes to a few tens of bytes, that can do, well, something unexpected. Such viruses attach themselves to executable files— programs, so that the infected program, before proceeding with whatever tasks it is supposed to do, calls the virus code. One of the simplest ways to acplish that is to 5 append the virus code to the end of the file, and insert a mand to the beginning of the program file that would jump right to the beginning of the virus code. After the virus is finished, it jumps back to the point of origination in the program. Such viruses were very popular in the late eighties. The earlier ones only knew how to attach themselves to .Com files, since structure of a .COM file is much simpler than that of an .EXE file—yet another executable file format invented for MSDOS operating system. The first virus to be closely studied was the Lehigh virus. It attached itself to the file that was loaded by the system at boot time—. the virus did a lot of damage to its host, so after threefour replications it was no longer usable. For that reason, the virus never managed to escape the university work. When you execute program code that?s infected by a virus, the virus code will also run and try to infect other programs, either on the same puter or on other puters connected to it over a work. And the newly infected programs will try to infect yet more programs. When you share a copy of an infected file with other puter users, running the file may also infect their puter。 an av program that hasn?t been updated for several months will not provide much protection against current viruses. 2. In addition to scanning for viruses on a regular basis, install an ?on access? scanner (included in most good av software packages) and configure it to start automatically each time you boot your system. This will protect your system by checking for viruses each time your puter accesses an executable file. 3. Virus scans any new programs or other files that may contain executable code before you run or open them, no matter where they e from. There have been cases of mercially distributed floppy disks and CDROMs spreading virus infections. 4. Antivirus programs aren?t very good at detecting Trojan horse programs, so be extremely careful about opening binary files and Word/Excel documents from unknown or ?dubious? sources. This includes posts in binary newsgroups, downloads from web/ftp sites that aren?t wellknown or don?t have a good reputation, and executable files unexpectedly received as attachments to Email. 5. Be extremely careful about accepting programs or other flies during online chat sessions: this seems to be one of the more mon means that people 8 wind up with virus or Trojan horse problems. And if any other family members (especially younger ones) use the puter, make sure they know not to accept any files while using chat. 6. Do regular backups. Some viruses and Trojan horse programs will erase or corrupt files on your hard drive and a recent backup may be the only way to recover your data. Ideally, you should back up your entire system on a regular basis. If this isn?t practical, at least backup files you can?t afford to lose or that would be difficult to repla