Question: Host machine was ported from 11.10 to 12.04 LTS (Xubuntu)

Virtualbox image is win7 x64

With upgrade /dev/vboxdrv was missing, so within synaptics I reinstalled:

* virtualbox (4.1.12dsfg-2 not OSE)
* virtualbox-qt 4.1.12dsfg
* virtualbox-dkms 4.1.12dsfg

Then it works again but my windows is badly crashed. Once started and desktop icons pops out it's really slow and crash (network icon is also blocked)

I reinstalled guest additions, but does not make any difference.

Nothing is serious because I still have available snapshots.

I'm asking for advices, like for example how do I clean reinstall (clear parameters) vbox ?
 Is there any workaround, or do I need to wait fresh 12.04LTS updates to fix this pbm by magic ?

Answer: 
The [probably-related VirtualBox bug ticket](https://www.virtualbox.org/ticket/10528) says it is fixed in VirtualBox source, and also details a workaround via grub.

The Debian bug is [here](http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=655202).

Hopefully the patch will be applied to Ubuntu-12.04 .

(This answer is also at: <https://askubuntu.com/questions/130726/win-7-virtual-box-vm-fails-to-start-after-upgrade-to-12-04> )
I can't be sure that I was having the same problem, but I also just upgraded from ubuntu 11.10 to 12.04 and attempting to boot my virtual machine (xp 32bit) would cause the host to freeze.

The fix was to do the following:

* Enable absolute pointing device (almost certainly wasnt this that fixed it but hey ho!)
* Enable VT-x/AMD-V acceleration and nested paging (found under settings->System->Acceleration)
* Disable video acceleration (again dont think it was this but I don't fancy fiddling now it works)

Hope this works for you too.