VirtualBox is open source virtualization software similar to VMware. I wanted to run a virtual Ubuntu LAMP server for testing, but I ran into a problem with VirtualBox.
Ubuntu Server has its own kernel which has PAE (physical address extension) support. This is not supported in VirtualBox and prevents the virtual Ubuntu from booting. Using Ubuntu’s recovery mode it is possible to install another kernel which will work in VirtualBox.
After installing Ubuntu server in the virtual machine, boot off the CD again and select the Rescue a broken system option.