Even though the heading on that error popup is labelled Microsoft .NET Framework, this is not a problem with the Microsoft .NET Framework, it is usually a problem with some app set to start with Windows.
Open Task Manager (Ctrl + Shift + Esc).
Select the Start-Up tab.
Disable all apps that start with Windows, except any drivers or anti-virus in the list that you may have installed.
Restart (not shut down) your PC and check if that stops the error on startup.
If it does, enable the startup apps one at a time and restart your PC to try to find what app is generating the error.