A popular myth in the early days of computing was that anti-virus vendors deliberately put out malware in order stimulate sales of their products. There was (almost) certainly no truth in the story, though proof-of-concept viruses are sometimes used for scary marketing even if they remain in the research lab; and instances of poachers turning gamekeepers abound in the security world.

A conspiracy theory waiting to do the rounds (if it is not doing so already) is that freely-available tools that allow you to crack software registration or bypass trialware timeouts are deliberately infected with trojans by major application developers in order discourage their use.

It seems, however, that there is no need for software houses to be so devious. Cracking tools downloaded from P2P networks or warez sites are riddled with malware by the people that develop and post them, according to security firm Trend Micro. So why do something that is going to scare people from using your code?

"It's the only way they can make money," said a spokesman. "They can stick in a trojan that will allow them to steal information such as banking details."

Just visiting warez sites can set your PC security alarms ringing like cash registers at Christmas, according to the spokesman. And downloaded utilities are often contained in zip files, self-extracting or not, that can carry all manner of extraneous code.

The top three downloads, all commonly infected, include a tool to help you convert trial Nero CD burning software into a full version; another to extract Windows and Microsoft Office registration codes from a computer, possibly for resale on the black market; and one to generate a code that can be used to register Windows XP.

Hot on their heels are a utility that hides program windows, and one that endeavours to stop other code (such as anti-virus software) running. Trend points out that both these tools can be used to cloak the activities of the malware they come with.