In case you have installed new Guest Additions into your VirtualBoxed Windows 7
and all you got was black screen after reboot, here is the solution! Boot into
safe mode (reset virtual machine and you will get safe mode offered at boot)
and uninstall Guest Additions. Power off the VM. Check Enable PAE/NX checkbox
in System -> Processor VM Settings, start Windows, install Guest Additions and
voila! System is back again. Be sure to curse a lot during every step.
If you have questions/comments, feel free to conact me at my e-mail.