A survey has shown, that there has been a significant growth of the number of PCs infected by malicious software that hackers use to send spam.

About six million PCs around the world could be now affected, which is 30 per cent more than a year before. They have been used by hackers to form a so-called bot network to produce network events on a large scale without the PCs' users knowledge.

A Symantec report has shown, that one in three attacks have been launched from US-based PCs in the second half of 2006. While the number of hijacked PC has grown significantly, the number of servers controlling the bot network decreased by 25 per cent in the last year. 

Symantec security researchers' report shows that there is a consolidation of bot network owners taking place which aim at expanding their reach and improving infrastructure for launching attacks world wide. 

Ollie Whitehouse, Symantec's consulting services director, said: "This rise in the number of infected computers can certainly be attributed to the rise in the online population of countries like China and Spain, in Europe.

"There is almost an educational curve that the users and service providers have to go through. Unfortunately when certain countries go through rapid increases in connectivity and availability of technology that curve is not always kept up.

Symantec's professionals said that the online criminals are evolving into a better organised structure, they are even competing with other hackers by making their faces public.