One of the great things about VirtualBox is that it allows you to open up, mount, and run any hard drive from the other 3 major manufactures. VMWare, Microsoft, and Parallels. However one of the bad things is that there are no automated utilities that just make them work like some of the manufactures provide. And a common issue that I have always run into, especially when converting from VMWare to VirtualBox is this error.

STOP 0x0000007B INACCESSABLE_BOOT_DEVICE
This error above specifically relates to the fact that Windows really hates you just changing hard drive controllers all willy-nilly, which is essentially what is happening when you switch from one VM to another. Luckily I have found sort of a fail-safe that seems to work in most if not all cases for me when switching to VirtualBox.
Solution
Your default configuration in VirtualBox probably looks something like the following:

However to get an existing hard drive to boot, that has been created with another Hypervisor, I have found you need to have this as your setup to get Windows to boot initially.

The choice of PIIX4 as the Type also seems to be important, but play with the Type first if you fail boot with PIIX4, before giving up.
VirtualBox

epic, thank u dude (:
+100000
You sir are a scholar and a gentleman! There will be epic novels and stories written about your work!
I’m not worthy I’m not worthy – grovels!
Thanks bro, I was struggling with that one. Although I had changed from SATA to IDE Controller, I hadn’t changed the type to PIIX4… your post helped me resolve an annoying issue within 30 seconds. Have a beer on me
It didn’t work for me… I’m not sure if creating the virtualbox of the whole disk (instead of the windows partition) could be generating this inconvinient.
PD: I use Ubuntu 12.04 and I’m virtualizing an existent windows xp partition
Thanks
I almost gave up, may you have a wonderful day and much good karma.
Thanks mate, your post saved me loads of time!
Thank a lot mate!!!
Almost gave up this one
My selection for the controller type is greyed out. How do I change it?
Thanks in advance!
Thank you for posting this! Quick and easy fix, helped me out a lot. Thought my VHD was damaged.
Great suggestion!
At first I had mounted the volume under a virtual SCSI controller. After I changed this to a virtual IDE controller it suddenly started to work!
The real problem is in hard drive configuration. It cannot be SATA, it must be IDE.
Thanks a lot