But I am having problem to start the coreOS instances, with the following error:

Result Code: E_FAIL (0x80004005) Component: Machine Interface: IMachine {480cf695-2d8d-4256-9c7c-cce4184fa048}

My configuration is windows 8.1 with virtualbox, vagrant and git all installed as described in the guide.

I'd appreciate if anyone can help.

==> core-01: Checking if box 'coreos-alpha' is up to date... ==> core-01: Clearing any previously set forwarded ports... ==> core-01: Clearing any previously set network interfaces... ==> core-01: Preparing network interfaces based on configuration... core-01: Adapter 1: nat core-01: Adapter 2: hostonly ==> core-01: Forwarding ports... core-01: 22 => 2222 (adapter 1) ==> core-01: Running 'pre-boot' VM customizations... ==> core-01: Booting VM... ==> core-01: Waiting for machine to boot. This may take a few minutes... The guest machine entered an invalid state while waiting for it to boot. Valid states are 'starting, running'. The machine is in the 'poweroff' state. Please verify everything is configured properly and try again.

If the provider you're using has a GUI that comes with it, it is often helpful to open that and watch the machine, since the GUI often has more helpful error messages than Vagrant can retrieve. For example, if you're using VirtualBox, run `vagrant up` while the VirtualBox GUI is open.

Failed to open a session for the virtual machine coreos-vagrant_core-01_1408322570242_93586.

The virtual machine 'coreos-vagrant_core-01_1408322570242_93586' has terminated unexpectedly during startup with exit code 1.

Result Code: E_FAIL (0x80004005) Component: Machine Interface: IMachine {480cf695-2d8d-4256-9c7c-cce4184fa048}