Installing Ubuntu 8.04 LTS in VirtualBox

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:

1. Make sure the VM is stopped
2. Go to Settings -> General -> Advanced tab -> check "Enable PAE/NX"
3. Save the settings. The VM should boot now...

Found it here.