After upgrading VirtualBox to version 4.3.10, you may find some or all of your virtual machines won't start, with an error message to the screen, including the error code NS_ERROR_CALL_FAILED 0x800706be.
While this may happen on a Windows host, I've only ever noted it on Linux hosts - although considering I've only got a single Windows host running, it's not really surprising that I wouldn't have seen it. If you check the log of a non-starting VM, the last few lines will look like this:
00:00:02.018323 Activating Local APIC
00:00:02.018364 CPUM: SetGuestCpuIdFeature: Enabled APIC
00:00:02.018935 PIT: mode=3 count=0x10000 (65536) - 18.20 Hz (ch=0)
00:00:02.024902 Shared Folders service loaded.
00:00:02.051791 thread routine failed the initialization
The last line is the important part.
This seems to be caused by a failure to read the hard drive image file properly. Simply compacting the hard drive from the command line usually fixes it.
First, shut down all VirtualBox processes, killing them through the top command, if necessary. Then, use this command:
vboxmanage modifyhd VirtualBox\ VMs/<VM name>/<VM hard drive file.vdi> --compact
where <VM name> is the folder your VM is stored in, and <VM hard drive file.vdi> is the file containing the hard drive image for the affected VM.
This may take a few minutes, depending on the size of the virtual hard drive, but once it's finished, you should be able to restart VirtualBox, and your VM should start properly.