In Windows you press the shift key when you login to disable any startup scripts from running. Is there an equivalent action in Ubuntu?
No, this is not possible.
There are 2 sorts of scripts:
Both do not have a keyboard switch to turn them off or on on the fly.
Go to dash and search startup, some are hidden for security reasons so in the terminal type this
sudo sed -i 's/NoDisplay=true/NoDisplay=false/g' /etc/xdg/autostart/*.desktop
And then try the startup application.