In virtual box, right click your virtual machine and go to settings. Change settings to use the host's CD drive and enable boot from CD. Now insert Ubuntu live CD in your disk drive and start your virtual machine. It would boot from the CD/DVD and start live Ubuntu session. Mount your virtual harddisk and edit the sudoers file on the virtual harddisk. So if you mounted the virtual HD to /mnt/vhdd then the sudoers file will be at /mnt/vhdd/etc/sudoers.
Edit this file and add the following line to its end
brallan ALL=(ALL) ALL
Save this file and shutdown the virtual machine. Remove the CD from the drive and start the virtual machine. Things should be back to normal now and you should now have sudo rights for everything.
Sorry but I missunderstood the question. I thought the problem was with guest OS and not host. For host computer you can boot your computer from live CD and follow the same steps i.e mount your hdd and edit sudoers file and add the above mentioned line to to it to grant user brallan sudo rights.