I've discovered that it's not so easy to migrate existing Windows installations in VirtualBox.
In my instance I was trying to migrate a Windows XP image, but I was stopped by a BSOD after the boot of the OS, which prompted an error with intelppm.sys (but this problem may also be caused by processr.sys).
The reason for this crash is because these drivers are attempting to perform an unsupported operation inside of the virtual machine.
To solve this problem you can manually disable these drivers (with no negative side effect) by going to the following location in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Processor
Or
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Intelppm
And changing the 'Start' value to '4'.
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