In 1981, almost at the same time as Elk Kloner (the first virus for 8-bit processors) made its appearance, a new operating system was growing in popularity. Its full name was Microsoft Disk Operating System, although computer buffs throughout the world would soon refer to it simply as DOS.
The development of MS DOS systems occurred in parallel to the appearance of new, more powerful hardware. Personal computers were gradually establishing themselves as tools that people could use in their everyday lives, and the result was that the number of PCs users grew substantially. Perhaps inevitably, more users also started creating viruses. Gradually, we witnessed the appearance of the first viruses and Trojans for DOS, written in assembler language and demonstrating a degree of skill on the part of their authors.
Far less programmers know assembler language than are familiar with high-level languages that are far easier to learn. Malicious code written in Fortran, Basic, Cobol, C or Pascal soon began to appear. The last two languages, which are well established and very powerful, are the most widely used, particularly in their TurboC and Turbo Pascal versions. This ultimately led to the appearance of virus families: that is, viruses that are followed by a vast number of related viruses which are slightly modified forms of the original code.
Other users took the less artistic approach of creating destructive viruses that did not require any great knowledge of programming. As a result, batch processing file viruses or BAT viruses began to appear.
The development of 16-bit processors led to a new era in computing. The first consequence was the birth of Windows, which, at the time, was just an application to make it easier to handle DOS using a graphic interface.
The structure of Windows 3.xx files is rather difficult to understand, and the assembler language code is very complicated, as a result of which few programmers initially attempted to develop viruses for this platform. But this problem was soon solved thanks to the development of programming tools for high-level languages, above all Visual Basic. This application is so effective that many virus creators adopted it as their daily working tool. This meant that writing a virus had become a very straightforward task, and viruses soon appeared in their hundreds. This development was accompanied by the appearance of the first Trojans able to steal passwords. As a result, more than 500 variants of the AOL Trojan family -designed to steal personal information from infected computers- were identified.
More information about the computer threats mentioned here and about other threats is available from Panda Softwares Virus Encyclopedia--