I just ran into this problem too, running the latest VirtualBox ver. 4.3.0.r89960 on OSX 10.9.
Everything had been running fine, then there seemed to be some sort of network glitch, and my macbook froze while running a VM. I had never seen it do that before, and after a hard reboot, I started getting the kernel driver not installed error whenever I tried to start a VM.
I tried the restart command, but I still got the same error. Finally I just re-installed VirtualBox from the same dmg I had used before, and that fixed the problem, my VMs are running fine now.