Adware rather than spyware is the most common problem code on PCs, according to a recent survey of infected PCs.

The research, conducted by privacy company Webroot, found that seven of the top 10 types of malware were related to advertising, usually redirecting searches or monitoring user activity to display targeted advertising. Two of the 10 are re-diallers and one is a key-logger for stealing security codes.

"Our most recent research shows that spyware writers are continuing to innovate and find new, more deviant ways to infiltrate systems," said Richard Stiennon, vice president of threat research at Webroot.

"The increased presence of hijackers, diallers and key-loggers in our findings demonstrates that the new trend is to go straight for the jugular. CoolWebSearch, a particularly nasty hijacking program with dozens of iterations, was found on nearly 10 per cent of all audited systems."

CoolWebSearch and the rest of the top five in Webroot's list were all advertising-based. This malware is designed to divert web searches to other domains to drive up traffic for specific sites or display certain adverts based on computer users' activities.

More worrying is the presence of two rogue diallers in the list. BT recently took action against diallers but they are still a major problem.