To confirm that the process running on your PC is legitimate, follow these steps:
A pop-up error stating "The instruction at 0x... referenced memory at 0x... The memory could not be read."
If you never installed F-Secure and you see fsmainui.exe running, run a full system scan with Windows Defender or a second-opinion scanner like Malwarebytes. A legitimate F-Secure file does not appear on a clean Windows installation without the parent software.
If you'd like, I can: