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OS crash bug
Windows 7 64bit SP1
VirtualBox Version 4.1.2 r73507
VirtualBox Version 4.1.0 r ?????
Confirmed

Start the VirtualBox in Windows Virtual PC
Falls involving the OS when you start the guest OS.
VirtualBox falls into when you start Windows Virtual PC or VirtualBox running.

SYSTEM_SERVICE_EXCEPTION
0x0000003b (0x00000000c0000096, 0xfffff88009876972, 0xfffff88008b37e90, 0x0000000000000000)

VBoxSVC.log-----------------------------------------

VirtualBox (XP)COM Server 4.1.2 r73507 win.amd64 (Aug 15 2011 14:29:50) release log
00:00:00.016 main Log opened 2011-08-28T06:29:02.256769400Z
00:00:00.016 main OS Product: Windows 7
00:00:00.016 main OS Release: 6.1.7601
00:00:00.016 main Executable: C:\Program Files\Oracle\VirtualBox\VBoxSVC.exe
00:00:00.016 main Process ID: 2492
00:00:00.016 main Package type: WINDOWS_64BITS_GENERIC
00:00:00.016 Loading settings file "C:\Users\XXXX/.VirtualBox\VirtualBox.xml" with version "1.12-windows"
00:00:01.092 VDInit finished
00:00:01.108 Loading settings file "C:\Users\XXXX\VirtualBox VMs\PWPS\PWPS.vbox" with version "1.12-windows"
00:00:12.012 ERROR [COM]: aRC=VBOX_E_INVALID_VM_STATE (0x80bb0002) aIID={5eaa9319-62fc-4b0a-843c-0cb1940f8a91} aComponent={Machine} aText={Machine is not locked for session (session state: Unlocked)}, preserve=false
00:00:12.340 ERROR [COM]: aRC=VBOX_E_IPRT_ERROR (0x80bb0005) aIID={5eaa9319-62fc-4b0a-843c-0cb1940f8a91} aComponent={SessionMachine} aText={Saved screenshot data is not available (VERR_NOT_SUPPORTED)}, preserve=false

Start the VirtualBox in Windows Virtual PC

Nested virtualization is not supported.

Well... I run into same problem / error running VirtualPC and VirtualBox simultaneously, because I need on same host PC to run some tests on both virtualization technologies.
VirtualPC and VMWare at same time have no problem... just with VirtualBox (4.1.22).

There is any chance to run both of them in parallel?...

With no knowledge of the guests or host, who can know? However I can say that if they both require VT-x then no - you can't run them together.