Windows 7 users have been advised to upgrade their PCs to prevent possible cyber attacks as software support ends.
As of Tuesday, the 10-year-old operating system will no longer receive critical updates, meaning Microsoft will no longer fix any weaknesses that appear.
PCs will eventually become vulnerable to hacker attacks.
Windows 7 is still one of the most popular Windows operating systems with a 32.74% global share, according to NetMarketShare estimates.
Only loses the latest version of Microsoft, Windows 10, with 47.65%.
GCHQ's National Cybersecurity Center (NCSC) has warned the public not to access banking, email or other services that contain sensitive device information in Windows 7 due to the high risk.
Ending support is a long-term measure by Microsoft as it changes old technologies and focuses on newer technologies.
An NCSC spokesman said: "NCSC would encourage people to update devices currently running Windows 7, allowing them to continue receiving software updates that help protect their devices.
"We recommend that software users after the deadline replace unsupported devices as soon as possible, move sensitive data to a supported device, and not use it for tasks such as accessing banks and other sensitive accounts.
"They should also consider accessing email from a different device."
Affected computers will remain functional but no longer secure.
Companies that are not ready for the move may choose to pay Microsoft for extended security updates through January 2023.
However, it is sold by device and the price will increase every year in an attempt to encourage companies to upgrade.