All right, then how about this:
Malware, such as computer viruses, worms, and Trojan Horses, are small programs designed to infiltrate an unsuspecting user's computer, copy themselves over and over, and use that computer as a launching point to infect other computers.
So, some guy wirtes this tiny little program. It takes advantage of flaws in a software program, be it a browser, email reader, or even a word processing program to insert itself into the users hard drive and take over the system.
Some of these programs are also designed to copy personal information to be sent back to the maker-- credit card numbers, account numbers and passwords, etc. Or it may set up the computer to be used as a zombie for concerted denial-of-service attacks against large internet servers, or to spread more malware, to upload illegal software to the internet such as hacked programs or kiddie porn.
So, not only does it consume resources on your computer, it also uses your computer to wreak further havoc, not to mention robbing you of money when someone steals your identity.