VirtualBox stuck on "Starting Virtual Machine"

Please check if the VBoxSVC service is still running when you try to start the VM with VBox 4.3.18.

Make sure that you have the use I/O APIC, and the Use host I/O cache options are checked.

Also you may want to upgrade to 4.3.20, since it have a lot of fixes over 4.3.18.

This is a missing feature in VirtualBox and will be fixed in future releases. Please add nosmap to the kernel parameters of the host kernel. To follow this problem, see VirtualBox ticket 13820.