Millions of computers around the world are - without the knowledge of their owners - under the control of cybercriminals who commandeer the PCs to send spam, distribute malware, and commit identity theft, Sophos said. Billions of spam messages are sent every day, with over 99% determined to be relayed from innocent users' computers that have been hijacked and turned into a zombie.

"If your computer has been turned into a zombie then a hacker has complete control of your PC. That means they can read everything you type, see every website you visit, as well as spam out adverts through your internet connection without your knowledge", Graham Cluley, senior technology consultant for Sophos said.

"Next time you receive spam in your inbox remind yourself this - that an advert for Viagra could have been sent from a little old lady's computer in Skegness, and she doesn't know anything about it. It's all too easy to be part of the spam and malware problem, but it's time for people to take a stand.

"The answer is to keep your computer's security up-to-date with anti-virus software, security patches and firewalls. We need more people around the world to wake up to this truly global problem."

"With nights getting longer and colder, many people will be at home this weekend and Halloween would be the perfect opportunity to scare hackers off your computer by scanning it for threats", Cluley added.

A video illustrating the launch of Sophos' Kill-A-Zombie Day can be found on YouTube.

To illustrate the global zombie problem, Sophos has produced a chart of the top 12 spam-relaying countries.