Virus

A virus is a computer code or program, which is capable of affecting your computer data badly by corrupting or destroying them.
Computer virus has the tendency to make its duplicate copies at a swift pace, and also spread it across every folder and damage the data of your computer system.
A computer virus is a malicious software program or "malware" which, when infecting your system, replicates itself by modifying other computer programs and inserting its own code.

Disrupts the normal functionality of respective computer system.
Disrupts system network use.
Modifies configuration setting of the system.
Destructs data.
Disrupts computer network resources.
Destructs of confidential data.
Infected computer programs may include data files, or even the "boot" sector of the hard drive.

A very slow running computer
Pop-up messages that appear out of nowhere and are hard to get rid off.
Computer programs starting that you don’t recognize or didn’t start up yourself.
You can hear the sound of the hard drive in constant action.
Computer does not take command properly.