Vagrant issue

Hi
I'm trying to set up a nodejs environment to fly my AR Drone following your instructions.
When I come to the Vagrant part, I get

Warnings:
- `config.vm.customize` calls are VirtualBox-specific. If you're
using any other provider, you'll have to find provider-specific configuration
to translate to manually in your Vagrantfile.
Bringing machine 'default' up with 'virtualbox' provider...
There was an error while executing `VBoxManage`, a CLI used by Vagrant
for controlling VirtualBox. The command and stderr is shown below.

Command: ["list", "hostonlyifs"]

Stderr: VBoxManage.exe: error: Failed to create the VirtualBox object!
VBoxManage.exe: error: Code CO_E_SERVER_EXEC_FAILURE (0x80080005) - Server execution failed (extended info not available)
VBoxManage.exe: error: Most likely, the VirtualBox COM server is not running or
failed to start.

VirtualBox is installed on my computer (I'm running Windows 7)
Could you please give me a hint about the kind of problem I have ?
Thanx