The message, which was spotted by InfoWorld, tells the user that Windows 10 is "free for a limited time" before displaying two big buttons. There is no option to permanently postpone the upgrade process beyond a small red cross in the top right corner.
Microsoft has previously been downloading large primer files for Windows 10 onto computers ready for a potential upgrade, even if the user had never given any intention of wanting the new operating system.
The company has been pushing users to upgrade to Windows 10 for some time now, but the no-way-out pop-up window is a new development.
Microsoft has been forced to reassure users about other features within Windows 10, such as the operating systems constant connectivity to the cloud. The company has released software updates to fix these issues.