It could be the biggest April Fool's joke ever played on the internet, or it could be one of the worst days ever for computers connected to the network. Security experts can't work out whether the Conficker virus – which has infected more than 10m Windows PCs worldwide – will wreak havoc on Wednesday , or just let the day pass quietly.

Experts have worked out that from midnight on 1 April, the Conficker program will start scanning thousands of websites for a new set of instructions telling it what to do next. The infected machines thus comprise one of the biggest "botnets" – a network of "robot" computers – in internet history. And if they were all given a target, such as simultaneously sending search queries to Google or trying to connect to a gambling site, they could knock it offline through the sheer volume of connections – a "denial of service". Victims usually discover that they have been locked out of their computers or have very slow-running internet connections.

It could be the biggest April Fool's joke ever played on the internet, or it could be one of the worst days ever for computers connected to the network. Security experts can't work out whether the Conficker virus – which has infected more than 10m Windows PCs worldwide – will wreak havoc on Wednesday , or just let the day pass quietly.

Experts have worked out that from midnight on 1 April, the Conficker program will start scanning thousands of websites for a new set of instructions telling it what to do next. The infected machines thus comprise one of the biggest "botnets" – a network of "robot" computers – in internet history. And if they were all given a target, such as simultaneously sending search queries to Google or trying to connect to a gambling site, they could knock it offline through the sheer volume of connections – a "denial of service". Victims usually discover that they have been locked out of their computers or have very slow-running internet connections.