Security experts have urged users running Windows 7 to migrate away from the software as Microsoft stops supporting it as of Tuesday.
British national security authorities also urged Windows 7 users not to use the system for sensitive tasks such as accessing online banking or email.
Microsoft said Windows 7 computers would continue to function after Tuesday, but would no longer receive technical support, software updates or security fixes.
“While you could continue to use your PC running Windows 7, without continued software and security updates, it will be at greater risk for viruses and malware,” Microsoft said in a blog post.
The software maker said Windows 10, the current version, can be installed on older systems but “it is not recommended”.
One in four Windows users is on Windows 7, according to StatCounter.
The National Cyber Security Centre, an arm of GCHQ, said users should avoid using Windows 7 to access sensitive data, The Telegraph first reported.
“We would urge those using the software after the deadline to replace unsupported devices as soon as possible, to move sensitive data to a supported device and not to use them for tasks like accessing bank and other sensitive accounts,” the NCSC said.
“They should also consider accessing email from a different device.”
The security agency pointed out that hackers began exploiting flaws in Windows XP soon after Microsoft stopped supporting the system in 2014.
Businesses reliant on Windows 7 can pay Microsoft to continue to receive updates for Windows 7 Professional or Windows 7 Enterprise, in a programme set to continue for the next three years.
Charges range from $25 (£19) to $200 per device and are to increase each year.
Microsoft’s latest operating system, Windows 10, costs £120 but also requires relatively new hardware.
Windows 7 was first released more than ten years ago, in 2009.
Microsoft said it would be risky to continue using the software.
“If you continue to use an unsupported version of Windows, your PC will still work, but it will become more vulnerable to security risks and viruses,” the company said in a statement.
“Your PC will continue to start and run, but you will no longer receive software updates, including security updates, from Microsoft.”
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