So I have a Xubuntu box, an old PC that was running Windows previously. All is peachy with the Linux install bar one thing: every time I shutdown the system, it shuts down fine, but immediately restarts again. This wasn't happening with Windows and appeared to be an issue particular to Linux or Ubuntu.
Do you have a USB keyboard? Does it work fine? Does it also work fine once you boot into your operating system? Does it not work at all during the boot process before loading your operating system?
If your answer is yes to all of the above, then you've very likely been an idiot like yours truly and enabled the Fast Boot feature in your Intel motherboard's BIOS without reading the fine print.
Using Virtualbox (4.3.6) on an XP64 machine, I ran into the following error while trying to install a 64-bit version of Ubuntu:
this kernel requires an x86-64 CPU, but only detects an i686 CPU, unable to boot
This happens even if the host machine is a 64-bit machine. To fix this, reboot the box and enter the BIOS. Look around for a virtualisation setting and enable it. Save and exit the BIOS configuration screen.