For instance, I routinely run Virtualbox and NMware Desktop simultaneously. But if I forget and start XP mode while either or both are running---BOOM! BSOD.
I doubt anyone cares, but I thought I should clarify. My experience has been thus: If you are running VPC and you start Virtualbox, vbox checks something and then prints a warning. On the other hand, if vbox is running and you start VPC, BSOD.
This just supports my blaming VPC, because it's not a good neighbor, in this case.
It doesn't help OPs problem, I just thought I would throw it out there. It may not be true anymore, it could be fixed.