Whenever I try to install this program that I paid for, this comes up ''Setup can not continue because important files are missing'' Ive tried everything.
But I have windows 8.1, do I need windows 10? Or whats the problem?
Windows 8.1 should be fine, but please check that your Windows Updates are up to date, at least for the critical / important ones.
If that isn't the issue, then you will probably need to remove F-Secure completely, using the F-Secure Uninstallation Tool, reboot your machine, and start the installation again. It's possible that the installation failed for some reason the first time, and has left some files behind, which may be causing your issues.