IMAGINE it. Zombie computers that are possessed by the spirits of others. Zombie computers that infect new victims in cyberspace. Zombie computers hell-bent on attacking our interests and stealing our identities.

They exist, and there is a good chance your PC is one of them. But the fightback has begun. And in a report published last week, computer scientists in Germany have revealed details of the progress that the counter-attack has made.

It makes for inspiring reading. On our behalf, programmers are coming up with ingenious ways to observe, trap and eliminate zombies. But there is also evidence that the zombies are becoming cleverer and more evasive. At stake is the security of large swathes of the internet, for the zombies' creators are determined to hijack our PCs and use them to steal our personal details, send out spam and derail online businesses.

Zombies are created when hackers use a ...