Can't Install VBoxGuestAdditions

[f4]

I have Virtualbox 3.1.8 on Windows 7 and I created an Ubuntu VM, however it's impossible to install Virtualbox Guest Additions, in fact I receive this Virtualbox error message:

Unable to unmount the CD/DVD image C:
\PROGRA 1\SUN\VIRTUA 1\VBoxGuestAdditions.iso
on the machine "name of the ubuntu VM here"

Could not mount the media/drive 'C:
\PROGRA 1\SUN\VIRTUA 1\VBoxGuestAdditions.iso
(VERR_PDM_MEDIA_LOCKED).

Details:

Result Code:
E_FAIL (0x80004005)
Component: Console
Interface: IConsole {6375231a-c17c-464b-92cb-ae9e128d71c3}
Callee: IMachine {99404f50-dd10-40d3-889b-dd2f79f1e95e}


Under this message there is a button "force unmount" but even pressing it nothing gets solved.

[mpack]

"Media Locked" usually means that you created a snapshot or saved state while the ISO was mounted. In the latter case you need to run the VM and then shut it down fully, or right click and "Discard saved state". If the problem is snapshots then you need to delete them.

[f4]

First, I didn't create any snapshot.
Second, when I shut down the VM, I select that VM on the Virtualbox panel but the option "Machine->Discard" is not active...