Trojan-horse software is designed to perform unwanted tasks on a computer, while masquerading as a useful application. They allow malicious software to be installed on a machine via a ‘back door’. Examples include downloadable software which claims to remove malware from a computer but actually installs it without the user’s knowledge. Data compiled by BitDefender in the first half of 2009 showed that Trojans accounted for some 83% of all malware on the Internet.