Windows VMs Might Require I/O APIC

If you're virtualizing Windows in VirtualBox, you might have experienced a problem that causes the guest OS to completely fail to boot. If you know the installation to be non-corrupted, the issue is probably caused by I/O APIC being set incorrectly in the hypervisor.

APIC - Advanced Programmable Interrupt Controllers - are a CPU feature that requires special support in Windows. If Windows was initially installed with I/O APIC present, I/O APIC must be on when that OS is running. Conversely, if APIC was not present at installation, it must not ever appear. Windows, starting in Windows 2000, installs very different kernel files depending on the presence of APIC. APIC is required for 64-bit Windows OSes or if you want to use multiple CPUs.

Read more at this ServerFault question or the Wikipedia entry on APIC.