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ActiveX controls and Java classes Boot sector virus File-infecting virus Macro virus Multi-partite virus Script virus Trojan horse program Worm Encrypted code signatures Mutating code signatures Polymorphic code signatures Stealth techniques Broadly speaking, viruses fall into two general categories: “boot sector” and “file-infecting” viruses. Boot viruses dwell in the boot sector of the hard or floppy disk that carries them. These execute as your computer starts or whenever your computer reads information from an infected floppy disk. Once they copy themselves into your computer’s memory, they can then spread to other disks or other computers on a network, each time leaving copies of themselves that can repeat the cycle. File-infecting viruses become active only when you execute the program that carries them or, in the case of macro and script viruses, when a program opens a data file that carries them. Typically, such viruses infect files with extensions such as .EXE, .COM, or .DLL, and non-executable files such as Microsoft Word .DOC files or Excel .XLS data and template files. A huge increase in the variety and scope of new viruses over the last few years, however, has expanded this list to cover nearly any type of file—even some plain text files in certain circumstances. Once it executes, the file virus also loads itself into your computer’s memory, replicates, then attaches itself to other executable programs. “malicious software” or “hostile code.” Other agents have worm-like or Trojan-like characteristics, but come in the form of Java applets or ActiveX controls. Anti-viruses software detects these and other forms of malicious software, all at the same time, to provide you with complete protection against harmful software agents.