Don't do anything not mentioned in the tutorial.
Reinstall, and then follow the instructions in User Manual section 1.7: Creating your first virtual machine. So, I would start again: uninstall VirtualBox, delete the \.VirtualBox folder. You could undo this damage, but I don't know what else you've done. This error is terminal, hence 'Critical error'. The error occurs when VirtualBox tries to create a VM folder in your Downloads folder, since of course an ISO file and a VM folder can't both use the same name. It also sounds like you changed your default machine folder to that of your Downloads folder. It sounds like you are trying to give the VM the name of your boot ISO file, which isn't a good start.
