|Microsoft has issued a warning about a serious vulnerability in all versions of its Internet Explorer (IE) browser.
If exploited by a booby-trapped webpage the bug would allow attackers to take control of an unprotected computer.
Code to exploit the bug has already been published though Microsoft said it had no evidence it was currently being used by hi-tech criminals.
A workaround for the bug has been produced while Microsoft works on a permanent fix.
The bug revolves around the way that IE manages a computer's memory when processing Cascading Style Sheets - a widely used technology that defines the look and feel of pages on a website.
Hi-tech criminals have long known that they can exploit IE's memory management to inject their own malicious code into the stream of instructions a computer processes as a browser is being used. In this way the criminals can get their own code running and hijack a PC.
Microsoft has produced updates that improves memory management but security researchers discovered that these protection systems are not used when some older parts of Windows are called upon.
In a statement Microsoft said it was "investigating" the bug and working on a permanent fix. In the meantime it recommended those concerned use a protection system known as the Enhanced Mitigation Experience Toolkit.
Installing and applying the toolkit may require Windows XP users to update the version of the operating system they are using. But even if they do that some of the protection it bestows on Windows 7 and Vista users will not be available.
"We're currently unaware of any attacks trying to use the claimed vulnerability or of customer impact," said Dave Forstrom, the director of Microsoft's Trustworthy Computing group, in a statement.
"As vulnerabilities go, this kind is the most serious as it allows remote execution of code," said Rik Ferguson, senior security analyst at Trend Micro, "This means the attacker can run programs, such as malware, directly on the victim's computer."
He added: "It is highly reminiscent of a vulnerability at the same time two years ago which prompted several national governments to warn against using IE and to switch to an alternative browser."
|Low-cost computers are to be offered as part of a government scheme to encourage millions of people in the UK to get online for the first time.
Prices will start at £98 for a refurbished PC, with subsidised net connections available for £9 a month.
The 12-month trial is part of the Race Online 2012 scheme, which aims to reach out to the 9.2 million adults in the UK who are currently offline.
Distributor Remploy hopes to sell 8,000 machines in the next 12 months.
"Motivation and inspiration are still two of the biggest barriers [to using the internet], but clearly perception of price is another big deal for people," Martha Lane Fox, the UK's digital champion, told the Financial Times. "A good price point is certainly part of what helps people get online."
Race Online 2012, which aims to "make the UK the first nation in the world where everyone can use the web", estimates that of the more than nine million adults in the UK who are currently not online, four million are socially and economically disadvantaged.
The cheap computers will run open-source software, such as Linux, and will include a flat-screen monitor, keyboard, mouse, warranty, dedicated telephone helpline and delivery.
The packages will be sold through 60 UK online centres which offer IT training and Remploy, an organisation that specialises in helping disabled and disadvantaged people find work and which runs the computer recycling scheme e-cycle.
Race Online 2012 has also negotiated cheap internet packages using a mobile dongle, costing £9 a month or £18 for three months, to help people access the web.
Its research suggests that going online can save people around £560 a year and that thousands of jobs are offered exclusively online.
But the cost of owning and running a computer and net connection is often seen as a barrier for many people.
As a result, there have been several previous government-sponsored initiatives that offered cheap PCs.
The £300m Home Access Scheme began to distribute free laptops to pupils from poor backgrounds in January 2010. It was scrapped by the coalition government eight months later.