There are a lot of problems with the new VirtualBox 2 series. I've found many things don't work as they're supposed, I get lockups for no reason and the new features just don't warrent a full version increase. I've rolled back all my projects at work to VirtualBox 1.6.6 and decided in non-mission critical situations (home use) I'd continue using VirtualBox 2.02 and just deal with the bugs. Well, I'm officially back-pedaling on that now. VBox 2 is buggy as heck and I'm not going to continue using it.

In order to downgrade your VMs from 2.x to 1.6.6 you'll need to edit a few xml files. If you had 1.x installed and upgraded to 2.x then it will have molested your xml files so you'll need to edit them. For each VM there is an xml file in the Machines directory containing the configuration data.

/home/grant/.VirtualBox/Machines/Linux Mint/Linux Mint.xml

 <HardwareVirtExNestedPaging enabled="false"/>
        <GuestProperty name="/VirtualBox/GuestAdd/Vbgl/Video/SavedMode" value="1280x960x32" timestamp="1224693483507107000" flags=""/>

Oddly enough the open tag may not exist. I've seen this several time where only the close tag was present just solidifying the fact that VirtualBox 2.0 is half baked.

In order to get your VMs to run under VirtualBox 1.6.6 you need to delete all these lines in the xml file for each VM. If there are additional lines between and delete those as well. Once that's done restart VirtualBox 1.6.6 and they should start up just fine.