Over my experience level but decided to share.
Someone having a similar issue (Not vicidial related)
"Not sure if related to My other issues."
Just replaced my AMD Phenom II X2 550 + AMD 790GX board with an Intel i7-2600 CPU + Intel H67 chipset motherboard.
Built a new kernel (2.6.38), and done all that stuff to get Linux back up and running, and Virtualbox 4.0.4 working.
Linux VMs work just fine (CentOS, etc.). However, my XP VMs are inoperable. They just freeze during the bootup process, and don't come back. I can, however, boot them up in "Safe Mode".
Anyone running the latest/greatest new Intel chip, + Virtualbox, and running into this trouble with WindowsXP VMs specifically?
I've even done an Acronis "universal" restore of a VM backup, to no avail. Still get the freezing during bootup issue.
Oh when I go to poweroff the VM, this is what I get:
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root@linuxbox:~# VBoxManage controlvm "XP_Virt" poweroff
0%...10%...
And VBoxManage just sits there and hangs. Eventually I have to go in and manually kill the Linux process that is associated with it.
Not seeing anything in the bugtracker yet, but as this is a mixed Windows/Linux thing, I haven't a clue where to start in diagnosing. Don't know whether this issue belongs in the Windows forum (as its a Windows guest), or in the Linux host forum (as its a Linux host/hypervisor).
SOLUTION
Ended up replacing linux kernel 2.6.38 with 2.6.38.2 (just released the other day!). My error log (kernel messages) basically was matching what was in the 2.6.38.2 changelog. So problem solved

. Seemed to be a Sandy Bridge-specific issue with race conditions, and nothing to do specifically with the Virtualbox code.