Viruses

A computer virus is a program which  is able to reproduce/copy itself when it is run. Mostly, computer viruses are run as part of a program. In Biology, viruses work in the same way and this is how Computer Viruses took their name. Also Computer Viruses can execute orders that may cause harm to your computer. For this reason, computer viruses affect security. They are part of Malware. Very often, the term Virus is also used for other kinds of malware, such as trojans horses and worms. Even though this is wrong, it may be difficult to tell the difference between different kinds of malware; they often occur together, and only an expert may be able to tell the difference

On the top there is a video which will tell you 

which were the top 5 Computer Virus of 2009

Information taken from http://support.microsoft.com/kb/129972