i kept getting this error in virtual box NS_ERROR_FACTORY_NOT_REGISTERED
i followed the advice at https://www.virtualbox.org/ticket/3672#comment:28
which is :
cd /usr/lib/virtualbox/components
ln -s ../VBoxDDU.so .ln -s ../VBoxREM.so .ln -s ../VBoxRT.so .ln -s ../VBoxVMM.so .ln -s ../VBoxXPCOM.so .