If you have Windows 7 64bit, don't update VirtualBox to v4.3.18 or v4.3.20. Most of your VMs won't start and instead you'll get an error message about can't load some DLL. It's a known problem and no solution but going back to the prior version. v4.3.12 Works fine.
I had to restore the system to make sure it's clean state and recover the VB.