April Fool's Day computer virus could cause internet chaos today

A virus that experts believe has wormed its way into millions of computers could choose today to attack.

The Conficker virus, which is thought to have infected up to 15million computers since last autumn, has so far lurked harmlessly - but it is said to be programmed to change the way it operates today.

No one knows what it will do - with some suspecting it is simply an April Fool's joke.

But there are fears the virus, which infects machines through the internet and via memory sticks, will be used to steal bank details, send spam emails or crash a major website by overloading it by redirecting all affected computers to it.

Conficker, which is also known as Downadup or Kido, is a 'worm' virus that exploits a gap in Microsoft Windows software.

Parliament, the Ministry of Defence and several NHS trusts have been infected, as well as home computers.

Experts are sceptical about whether today is the day the worm will turn, but urge the public to ensure their anti-virus software is up to date.

Security patches can be downloaded from Microsoft, which has offered a £175,000 reward to find the culprits.

One sign of already being infected is being blocked from visiting the websites of anti-virus companies such as McAfee.

Graham Cluley, of internet security company Sophos, advises people to rid their machines of the worm as soon as they can.

He said: 'There is no reason to believe that there will be any instructions for Conficker to receive on 1 April,' he said. 'They could just as easily be delivered on 2 April, 4 April, 25 May or never.

'This network is large and the whole world is going to be watching everything it does.

'You could argue it is a victim of its own success because it is going to be difficult for it to do anything under the radar.'

Mikko Hypponen, of antivirus software firm F-Secure, said: 'It is scary thinking about how much control a hacker could have. They would have access to millions of machines.'

How do I know if my PC has been infected?

Conficker finds vulnerable computers and automatically disables security services and blocks access to anti-virus websites.

You might be infected if your internet connection is running particularly slowly.

Find out whether you already have Conficker by using Microsoft, Symantec or a McAfee security tool.

How do I stay safe?

Make sure your PC is running the latest version of anti-virus software such as Norton or McAfee.

Make sure any USB devices you use on your PC are from trusted sources - Conficker downloads files when an infected USB is used on a new PC

Update your PC with all the latest ‘patches’ from Microsoft - available from Microsoft's website.

If your PC has been infected with the virus you can download Microsoft's Malicious Software Removal Tool from the website.