If you are getting the following error on installing Ubuntu 8.04 server on VirtualBox:
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, you need to enable the PAE support for the virtual machine.
Follow the screens:
1. Open the settings for the machine (Ctrl + S).
2. Check the Enable PAE/NX bit.