VirtualBox 4.1.20 Installation Failed

[VladimirAus]

Here's how it worked for me. I had:
* VirtualBox 4.1.20 (which I could not update to 4.1 or 4.2)
* MacOS 10.7.5

1. Make sure your VirtualBox is working, i.e. install the last working version.
I just ran 4.1.20 installer after unsuccessful upgrade.

2. Quit all applications including VirtualBox.
3. In Terminal run

sudo /Volumes/VirtualBox/VirtualBox_Uninstall.tool ; exit;

You should run uninstall script for THE SAME VERSION of VirtualBox you got running.
Terminal will ask for password and will give the following warning.

...
Failed to unload org.virtualbox.kext.VBoxUSB - (libkern/kext) kext is in use or retained (cannot unload).
...
Failed to unload on or more KEXTs, please reboot the machine to complete the uninstall.
Done.
logout

4. Reboot asap.

After that I was able to run 4.2.1 upgrade... Let me know if that worked for you...