Today I got stuck with a 0x000025 error while trying to install Windows 7 64bit in VirtualBox. It turns out that I/O APIC must be enabled, according to the documentation:
On any host, you should enable the I/O APIC for virtual machines that you intend to use in 64-bit mode. This is especially true for 64-bit Windows VMs.
So, that’s all folks. Go to Settings > System > Extended features and check Enable IO APIC.