Virus is a program that is loaded onto a computer without the user’s knowledge. It self-replicates and infects a computer. A virus corrupts or deletes data on the computer and spreads itself to other computers. It may even erase everything on the hard disk. Malicious is any software that enters into a computer and damages it without the knowledge of the user. Spyware is a type of malware that is installed on computers and collects information about users without their knowledge. Worm is a special type of virus that makes copies over and over and uses memory but cannot add itself to other programs. Trojan horse is a simple program that pretends to be a useful application while it always does something destructive. It is not self replicating. A Trojan can spread only when it is copied to another system.
A virus can be broadly classified into three categories: boot virus, program file virus and macro virus. Boot virus affects the boot sector of both hard disks and floppy disks. Program file viruses infect executable program files. Macro virus affects macros in Word documents. A virus can cause problems in a computer in many ways such as: deleting or damaging files, frequent hanging of the system, etc. If the computer is behaving strange, it could be infected or have malware installed. Antivirus software is a computer program that detects, prevents and takes action to disarm or remove malicious software programs, such as viruses and worms.