If you try to install Ubuntu 8.04 in VirtualBox (I've tried the server version, probably the same is true for the Desktop version) and you get the following error:
The complete Message is: The kernel requires the following features not present on the CPU 0:6 Unable to boot - please use a kernel appropriate for your CPU
Then the problem is that the installer decided to set up a PAE kernel and you don't have PAE enabled for the VM. There are two possible solutions: change the installed kernel (harder) or enable PAE for the VM (easier). To do this, perform the following steps:
- Make sure the VM is stopped
- Go to Settings -> General -> Advanced tab -> check "Enable PAE/NX"
- Save the settings. The VM should boot now...
Found it here.