Microsoft is facing calls to pay compensation to Windows 10 users after claims it broke some computers.
A survey by consumer watchdog Which? found more than half of those who downloaded the software said it "adversely affected their PC".
Here is what you need to know:
What is Windows 10?
Windows 10 is the latest upgrade by Microsoft which launched in July 2015.
How many people use it?
It has been installed on more than 350 million devices since its launch in July 2015.
What have people been complaining about?
Files being lost.
Email accounts no longer syncing.
Failure to recognise web cameras.
Failure to recognise some anti-virus software.
Printers, wi-fi cards and speakers no longer working with a PC.
Windows 10 automatically downloading.
What do Microsoft say about it?
Microsoft said the Windows 10 upgrade is a choice designed to help people take advantage of the most secure,and most productive Windows.
A spokesman added: "Customers have distinct options. Should a customer need help with the upgrade experience, we have numerous options including free customer support".
What do Which? say Microsoft should do?
Which? has now called on Microsoft to consider "honour consumers' rights" and pay compensation "where appropriate".
Alex Neil, Which? director of campaigns and policy added: "We rely heavily on our computers to carry out daily activities so, when they stop working, it is frustrating and stressful. Many people are having issues with Windows 10 and we believe Microsoft should be doing more to fix the problem".