I just upgraded my Virtualbox from 4.2.18 to 4.3.4 on my Windows 7 host, upgrade went smooth and without problem.
When i tried to start virtual machines i got VT-x is not available. (VERR_VMX_NO_VMX) error on all of my VMs.
My PC’s processor doesn’t have VT-x hence the “Acceleration” tab on VM’s Settings > System is greyed out so there is no way i can change it from GUI. Virtualbox does run well on version 4.2.8 so i find it strange that 4.3.4 give error that VT-x is not available.
To fix this i had to run commands from command prompt:
VBoxManage.exe modifyvm "<vmname>" --longmode off
Change <vmname> with the name of your VMs from the list in Virtualbox.