What is a Computer Virus?

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation.

A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.

Computer viruses are often spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it’s from and you are expecting it.

Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files.

Computer viruses also spread through downloads on the Internet. They can be hidden in illicit software or other files or programs you might download.

Courtesy :- Microsoft Corporation
http://www.microsoft.com/security/antivirus/whatis.aspx