I installed Oracle Virtualbox per the instructions on the Oracle website, but can't use XP or Ubuntu VMs after switching back to the Ubuntu standard version.

To switch back, I have edited /etc/apt/sources.list to remove the Oracle repository, did an apt-get update, removed everything with virtualbox in the name using Ubuntu Software Center, and then reinstalled virtualbox and (after an error message) virtualbox-dkms packages.

I still can't start my virtual machines. How can I use the ubuntu version of Virtualbox again? The Oracle version crashes on my machine. What are the 'extension packs' that the Virtualbox error message complains about? These are different from the 'guest additions', right?

Also, why does re-installing ubuntu Virtualbox in Software Center fail to produce a working virtualization environment?

here is the error message when starting XP in the Ubuntu version of virtualbox (Version4.1.2_Ubuntu r38459) :

Failed to open a session for the virtual machine XP, defaults.
The device helper structure version has changed.
If you have upgraded VirtualBox recently, please make sure you have terminated all VMs and upgraded any extension packs. If this error persists, try re-installing VirtualBox.     (VERR_PDM_DEVHLPR3_VERSION_MISMATCH).
virtualbox_prob.txt (END)

and the error for an Ubuntu virtual machine:

 Failed to open a session for the virtual machine Ubu32.
The device helper structure version has changed.
If you have upgraded VirtualBox recently, please make sure you have terminated all VMs and upgraded any extension packs. If this error persists, try re-installing VirtualBox. (VERR_PDM_DEVHLPR3_VERSION_MISMATCH).
Result Code: 
NS_ERROR_FAILURE (0x80004005)

Result Code: 
NS_ERROR_FAILURE (0x80004005)
Component: 
Console
Interface: 
IConsole {1968b7d3-e3bf-4ceb-99e0-cb7c913317bb}