Netsky-P is the number one virus of the year making up 22.6 per cent of the number of virus reported.
"2004 was the year of the Netsky - the first of more than 30 versions of this worm arrived on the scene in February and an astonishing five variants have made it into the annual top ten," said Graham Cluley, senior technology consultant at Sophos.
The year also saw a 51.8 per cent increase in the number of new viruses and Cluley said there is no sign of viruses becoming a thing of the past.
"Those responsible for writing malware are more active than ever before," said Cluley.
All of the top ten virus were aimed at 32-bit Windows systems and Sophos said virus writers were likely to continue targeting the operating system in 2005 and beyond.
Cluley said there was a lot of hype around virus affecting mobile phones but these threats continue to be low and advised users to focus on the biggest threat – viruses affecting Windows PCs.www.sophos.com