If you still use Windows 7, Windows 8 or Windows 8.1, you can upgrade your PC to Windows 10 for free. Support for Windows 7 ended on January 14, 2020, and support for Windows 8.1 will end on January 10, 2023.

Trying to make Windows 7 secure by using an antivirus is like trying to keep your home secure by making sure that all the windows are locked properly while the main door is kept open. We can still eat a food after its expiry date, but we don't because it is bad for our health. Just like that, using an operating system after its expiry date (end of support date) is bad for the health of our devices and the valuable data we have on them.

More info here: http://nawzil.com/UpgradeW10