virtualbox.org

Is it possible to boot up virtual machines (In this case, Ubuntu) created with VirtualBox application (Inside host, in my case Windows 7 Pro x64) so that I can select whether load VMs without having to boot W7X64 or after I have logged in?

No. VirtualBox is an application which runs under the OS. You cannot do anything with a vm until the host OS boots and the VirtualBox application starts.