Solo es un enlace: http://www.linuxformat.co.uk/forums/viewtopic.php?t=6438
Para evitar perder la información, como ya nos ha ocurrido bastante, repetimos el contenido aquí:
1. Start Virtual box and log into Ubuntu.
2. Hit the right ctrl key so you can get your mouse pointer outside the virtual machine.
3.Go to top of virtual window, click on devices then select “Install Guest Additions”
You will see a window pop up inside Ubuntu showing you that there are some new files mounted in a virtual CDROM drive. One of those files should be VBoxLinuxAdditions.runYou must run the file with some admin permissions so do that this way…
4. Click inside the Ubuntu screen again then go to Applications – Accessories then Terminal. The terminal window is where you will run the file from, but first we must navigate to the correct directory.
5. type this… cd /media/cdrom0 (then hit enter, there is a space after cd!)
6. next type… dir (You should see amongst the files displayed VBoxLinuxAdditions.run)
7. now type… sudo sh ./VBoxLinuxAdditions.run (yes, that is a full stop before the slash!)
after you hit enter and it has done its stuff, the files are now accessable from Ubuntu.
8. You now need to reboot the virtual machine or press Ctrl+Alt+backspace.
9. Log onto the Ubuntu desktop and this time go to System – Preferences then Screen Resolution. You should now have more options than the three low res options you had at the beginning of the day!
Debería dar buenos resultados de frente (probado con Ubuntu 8.04 en VirtualBox bajo Ubuntu 9.10)