There's little doubt the internet has changed our lives for the better. You can book holidays, do your banking and pay bills online. But do you realise how easy it is for a cyber-crook to take control of your computer without you even knowing it?

Imagine this! You go to book tickets at a reputable theatre web site. That site has been infected by mal-ware. By simply clicking on that site your computer can become a slave to a central computer and join millions of other infected computers in what's called a 'botnet'. Immediately it is possible for a criminal to steal information from you, including your bank and credit card details but the thief can do even more. Your computer could be used to send thousands of spam emails a day to people around the world. It could even be used to attack businesses, or even entire countries. And you wouldn't notice a thing.

It sounds remarkable but, as Four Corners explains, one in six computers in Australia is, or has been, part of a 'botnet' and a 'botnet' attack has already destroyed a major business. Other 'botnet' attacks have resulted in personal information being stolen and then sold on the international black-market. The result? Bank accounts are emptied and millions of dollars stolen.

Police told Four Corners they are now so concerned by this type of crime they have set up covert operations on the web to infiltrate illicit marketplaces.