Computer Virus is a malicious piece of code often designed so that it is automatically spread to other computer users. Computer Viruses can be transmitted as attachments to an e-mail note, as downloads, or be present on a diskette or CD. , or a program, that 'infects' other programs by embedding itself within that program. The infected program is executed, which in turn executes the virus, allowing the virus to replicate itself and spread to the next program. The source of the e-mail note, downloaded file, or diskette you've received is often unaware of the virus. Some viruses wreak their effect as soon as their code is executed; other viruses lie dormant until circumstances cause their code to be executed by the computer. The user, more times than not will not be aware of the infection. Damage can range from annoyance, to a complete system meltdown, where the user's hard drive and in some cases, BIOS information is erased, resulting in the system being rendered inoperable.