There are three main classes of PC viruses:
Boot-sector viruses, which account for about 5 percent of known PC virus strains, reside in a special part of a diskette or hard disk that is read into memory and executed when a computer first starts. The boot sector normally contains the program code for loading the rest of a computer's operating system (hence the name, a reference to lifting oneself up by one's own bootstraps). Once loaded, a boot-sector virus can infect any diskette that is placed in the drive. It also infects the hard disk, so that the virus will be loaded into memory whenever the system is restarted. Boot viruses are highly effective: even though there are fewer strains, they were for a time much more prevalent than file infectors were
File Infector Virus---Roughly 85 percent of all known viruses infect files containing applications such as spreadsheet programs or games. When a user runs an infected application, the virus code executes first and installs itself independently in the computer's memory so that it can copy itself into subsequent applications that the user runs. Once in place, the virus returns control to the infected application; the user remains unaware of its existence. Eventually a tainted program will make its way to another computer via a shared diskette or network, and the infection cycle will begin anew.
Macro viruses, are independent of operating systems and infect files that are usually regarded as data rather than as programs. Many spreadsheet, database and word-processing programs can execute scripts--prescribed sequences of actions--embedded in a document. Such scripts, or macros, are used to automate actions ranging from typing long words to carrying out complicated sequences of calculations. And virus writers have created scripts that insert copies of themselves in other documents. Macro virusescan spread much more rapidly than other kinds of viruses because many people share "data" files freely--consider several workers swapping drafts of a jointly written report. "Concept," the first macro virus observed in the wild, infected its first Microsoft Word document late in 1995 and is now the most prevalent virus in the world. Today more than 1,000 macro viruses are known.
As well as basic replication code, viruses can contain whatever other code the author chooses. Some virus payloads may simply print a message or display an image, but others will damage programs and data. Even those without malicious payloads can cause damage to systems whose configuration differs from what the virus designer expected. For instance, the "Form" virus, which usually produces only a slight clicking noise once a month, overwrites one disk directory sector in a way that is harmless to older PCs but lethal to newer ones that arrange disk information differently.
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