It would be no exaggeration to say that there is a cyber crime wave in recent years. Presently, viruses are the most common problems which are causing serious damage to computer system. Virus is a program or code that replicates and infects another programme, sector or document by inserting itself or attaching itself to that medium. The effect of virus is that it destroys or alters the data files and other programs. Except in rare cases, the virus does not damage the computer hardware. There are more than 5000 different strains of viruses across the globe. For instance, ‘Love Bug’ virus of May, 2000 caused severe damage to working internet sites. So also, the virus recently developed by Pakistan has defaced the Indian web-site. Generally, there are two main classes of viruses. The file infectors, which attach themselves to ordinary program files. File infectors can either be direct action or resident. A direct-action virus selects one or more of the other programs to infect each time the program that contains it, is executed. A resident virus hides itself somewhere in memory. The first time an infected program is executed, and thereafter infects other programs when they are executed. The second category of virus is boot-record infectors. These viruses infect executable code found in certain system areas on a disk, which are not ordinary files. Examples include Brain, Sonde, Azusa, Michelangelo etc. which are always resident viruses. However, there are certain viruses which are able to infect both, hence they are called “boot and file” virus.