Is it possible to use a virtual machine like a kind of dual or multi-boot?
I mean: The computer starts up directly in the virtual machine and the you can chose what kind of OS you want to use (after its been installed in the virtual machine).
This is probably not possible without the use of some kind of OS.
If not possibly, what is the best (fast, small and less-consuming) OS that I can use?
Here's an idea:
1) Install some light-weight Linux distribution (like Debian) as a host OS.
2) Install the virtualization product of your choice; for now I'll go on with VirtualBox.
3) Install the virtual machines you want in VirtualBox.
4) In your host OS create a new user account for each machine; for example, an account called win7 for Windows 7 virtual machine. Put a command like vboxmanage start win7 in that users ~/.bashrc file.
5) In your host OS configure your login manager to show all the active user accounts, so then you can just click the virtual machine you want start. Depending on your security needs, you might also enable passwordless login for those accounts, so you don't even need to enter password for starting up a VM.
This is precisely what high-performance "bare metal" VM systems like VMWare ESX/ESXi do. They load a really, really tiny operating system kernel, which can't do anything except run one or more VMs.
Other bare metal hypervisors include Microsoft HyperV, HP's Integrity VM, and the free Xen project (used by many hosting services).
Virtual machines run from within the operating system so what you want isn't possible. You have to boot the machine with the installed OS (Windows, Lunix, OSX, etc) then start the virtual machine.
It sounds like you want a dual (multi) boot system which is possible.
