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: 
I can confirm this bug on two of my Ubuntu 12.04 machines (i7 Desktop, Samsung Dual Core Laptop):

All my Windows (Win7, WinServer2003,WinServer 2008) 64bit guest machines which were working under Ubuntu 11.10 behave the same way after upgrading / reinstalling to Ubuntu Precise:

During guest bootup or shortly after bootup (sometimes I can even access the guest's desktop for a moment) the complete system host + guest freezes completly.

The Vbox.log doesnt contain any hint. Even a fresh install of Ubuntu Precise has the same error (with the same guest machines).

I have not found any combination of guest settings which prevents this freeze yet. Even using the OSE edition of Virtualbox 4.1.12-UbuntuR7724S has the same error.
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> )