The Conficker virus, which had infected an estimated nine millions computers by Thursday is spreading at a rate of one million machines per day according to US software protection firm F-Secure.

USB memory sticks infected with the virus trick users into installing the worm, according to researchers.

The "Autoplay" function, which searches for programs on removable drives and is part of Microsoft's Windows Vista and early versions of Windows 7 platforms, is being exploited by the hackers.

The virus infiltrates this process and tricks users into believing it is a folder that can be opened. When a user clicks this bogus option, the worm installs and buries itself deep into the operating system.

Once there it attempts to contact a number of websites set up by the hackers. From there a further program could be downloaded that could takeover the infected machine.

Users are advised to install the latest security updates from Microsoft and have up-to-date anti-virus software installed.