In addition, Click advised them on what steps to take to make their systems more secure. Most computers have protection systems that need to be switched on and kept updated to protect them against the evolving threat from hackers.
Machines can be compromised simply by visiting an infected web page or opening an e-mail containing a virus as an attachment.
Hackers exploit unprotected computers for valuable data such as banking and credit card details.
Criminals use botnets to send out thousands of spam messages, store stolen data, and fraud.
For instance, "phishing" e-mails which attempt to trick people into revealing their bank details are often routed through a botnet.
Users are normally unaware that their PCs are being controlled remotely by cyber criminals because there are almost no symptoms.
Greg Day from security firm McAfee explained that the people who control botnets are "very skilled professionals."
"We've seen this move from what used to be a hobbyist bit of fun into something now that is very professional," he said.
Hackers are keen to recruit new PCs to a botnet to create a resource that they sell or hire out to other cyber criminals.
But some networks of hijacked computers are of "much more value" than others, according to Mr Erasmus.
"Computers from the US and the UK go for about $350 to $400 (£254-£290) for 1,000 because they've got much more financial details, like online banking passwords and credit cards details," he said.
This report will be broadcast in this week's edition of Click on Saturday 14 March at 1130 GMT on the BBC News Channel.