Computer viruses are computer program files or harmless code which enters into the computer without the knowledge of user. They can replicate themselves and spread from one system to another. They affect the program and performance of the system. They can damage data files and another disk by occupying the space on disk and in main memory. Viruses infect another computer by modifying the contents and attaching a copy of itself thus leading to its replication. It is sometime hard to know how a risk must be dealt with before the authors know its consequences. So viruses should never be leaved on the system.