Windows XP users topped the list of victims cyber-thieves targeted, said Maik Morgenstern and Andreas Marx from the German AV-Test group, which rates and ranks security software.

"Malware writers go for the low hanging fruits because it's a lot easier to infect systems running on an old Windows XP operating system compared to brand-new Windows 8.1, with all its built-in security features," they said.

"We think we will see a lot of attacks for Windows XP within the next few months, but attackers will also always add exploits for other Windows systems just to catch those systems as well."