Microsoft has pulled support for Windows 7, meaning users will no longer receive crucial safety updates.
This is likely to leave many open to viruses, scams, and fraud - particularly when it comes to online banking.
Here, we look at the scale of the problem, and what you should do if you use Windows 7.
Windows 7 was launched in 2009, but it has since been eclipsed in popularity by Windows 10, launched in 2018.
Yet Windows 7 remains popular due to the widespread unpopularity of Windows 8 and 8.1 - the two versions that preceded Windows 10.
Despite its popularity, Microsoft stopped supporting Windows 7 on Tuesday, 14 January this year.
Statistics website StatsCounter estimates that seven in 10 computers in the UK use a Windows operating system, and one in six of these is running Windows 7. That's a huge number of people who will no longer receive vital safety support from Microsoft.
If you're unsure, you can see if you're running Windows 7 by pressing the Windows logo key and R and typing winver into the box. (It stands for 'windows version'.)
The government's National Cyber Security Centre (NCSC) has warned Windows 7 users not to use online banking or access sensitive information with their computers. It even warned against sending emails.
Without the prospect of future updates, any flaws or vulnerabilities in the system will be exposed forever. Regular security updates keep computers safe from hackers who can use these flaws to access and steal your data.
With a supported operating system, engineers will identify and fix any flaws as they are discovered, giving you added protection.
Since it's so important to keep your online banking information private, accessing your bank online from a Windows 7 computer is now too risky a prospect.
The NCSC has urged all Windows 7 users to upgrade their operating systems or replace their devices.
In the meantime, you'll have to use another device for your online banking, such as your mobile phone, or switch to in-branch or telephone banking.
To safely use online banking on any device, you need to use the latest versions of all apps and operating systems.
You can find out if your Windows 10 computer needs updating by pressing the Windows key and typingcheck for updates.
To see if a Mac is up to date click Software Update in System Preferences.
To check for updates on an Android phone open the settings app, tap System, then Advanced, then System update. For iPhones, it's Settings, General, Software Update.
You can see if your banking apps are up to date in the Google Play Store and Apple App store, though often these updates will be automatic.
Most online banking apps and websites now have built-in fraud warnings, helping you take care when sending money to new payees.
However, scammers' tactics are increasingly sophisticated, and convincing-but-fake emails, phone calls, and text messages are still common.
If you suspect a scam, ignore whatever the scammer is telling you to do, wait a while, and then contact the body they were pretending to be directly.