Running an OS that was installed with VMWare on a VMDK disk will not straightforward with Virtual Box. It might be a small but important change but what worked for me was to activate the I/O APIC. Running Windows 2008 Server worked, although I first booted in Safe Mode so I don’t know whether it would boot up in a regular mode. The next boot was OK, with OS installing different drivers, etc.
The issue with this migration is that Windows will require re-activation as the (virtual) hardware signature has changed.