A reformed ex-hacker gave Click Online's Spencer Kelly a demonstration of just how much damage a worm or virus can do to your home computer.
Jacques Erasmus makes his living advising on computer security, helping to write software to repel hackers.
Jacques Erasmus now uses his experience to fight the hackers
His extensive experience comes from a less honourable past: as a hobby, he used to be a hacker himself.
But he says that, unlike him, today's hackers do not just do it for fun.
"The new breed of hackers are driven by money. That's their main motivation, extorting businesses and other institutions.
So how do these people, who presumably have day jobs related to computers, get together and decide to form a group of hackers?
Jacques Erasmus says: "I think they mostly hang out in chat rooms and forums on the 'net, discuss hacking computer security and from these groups they'll find people that they think are suitable, with the right skill set, and they'll form a team, a crew."
Jacques wanted to demonstrate just how risky it is to connect an unprotected PC to the internet.
Within seconds of infection, our PC started downloading some strange programs
We set up a poor Windows XP machine with no firewall or anti-virus software.
Connecting it to the internet would be like throwing it into a lion pen with raw meat strapped to its hard drive.
How long would it be before we were hit by something nasty on the net? Hours, minutes?
As it turned out - eight seconds!
We were hit by Sasser, one of the fastest spreading worms on the internet.
It wastes no time at all in taking over your PC.
Within seconds of infection, our PC started downloading some strange programs, or payloads, from mysterious internet addresses.
These payloads are the programs that can take control of your machine and turn it into a remote controlled bot.
Our machine then started scanning random internet addresses, looking for other vulnerable PCs to infect.
Then Internet Explorer started downloading spyware.
Within five minutes our PC was running so many malicious programs that the CPU (central processing unit) was running flat-out at 100% - and we were not even touching it.
Large websites and businesses are frequently targetted by hackers
Perhaps the most sinister thing about an infected PC is that it can become part of a "botnet" - a network of seemingly innocent but infected machines whose combined processing power can be hired out to organized crime.
These botnets can comprise hundreds or thousands of zombie PCs, all awaiting instructions.
One of the most common instructions would be to launch a concerted attack on a popular website - a DDOS (distributed denial of service) attack, where major websites are flooded with repeated bogus requests from hundreds of zombie PCs.
Overwhelmed by the traffic, the site goes down.
Several large websites, including Google, have already fallen victim to DDOS attacks.
Jacques Erasmus explains: "First is the extortion, where they'll phone a high profile website that has lots of visitors and makes money, and they'll say to them 'give us £100,000 or we'll take down your website for X amount of hours'."
If the targeted website then fails to pay the money it is DDOSed to death.
Our PC crashed completely in less than 30 minutes
Another function of a remote-controlled PC is to report back all the keystrokes typed on its keyboard.
This is thought to be how hackers recently obtained passwords to the systems of the Sumitomo Mitsui bank in London, and began electronically stealing funds.
In this case, police foiled the scam and made an arrest.
But following the money trail often proves difficult, as Jacques points out.
"I've heard that these guys all set up Latvian bank accounts, which are pretty much untraceable. Latvia is the new Switzerland.
"If you found a hole in software that millions of people use, and is very high profile, you can sell that to the highest bidder for perhaps one or two million dollars."
Of course, if you have been a victim of hacking, it is no laughing matter.
It certainly was not for our PC, which crashed completely in less than 30 minutes.
It is interesting to note that although we were only hit by three worms in 25 minutes, the damage each of them did was enormous.
All of it could have been prevented with anti-virus software and a firewall.
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