Hum. This might be a false positive from driver verifier, not very sure though.

The BSOD was caused by Sonic CD/DVD driver (used by many different CD/DVD programs) but I would disregard this dump. Disable verifier and let us know if you get BSODs again.
Code:
*******************************************************************************
*                                                                             *
*                        Bugcheck Analysis                                    *
*                                                                             *
*******************************************************************************

Use !analyze -v to get detailed debugging information.

BugCheck C9, {228, fffff88001c62f74, fffff98004dd6d80, 0}

*** WARNING: Unable to verify timestamp for PxHlpa64.sys
*** ERROR: Module load completed but symbols could not be loaded for PxHlpa64.sys
Probably caused by : PxHlpa64.sys ( PxHlpa64+2f74 )

Followup: MachineOwner
---------

0: kd> !analyze -v
*******************************************************************************
*                                                                             *
*                        Bugcheck Analysis                                    *
*                                                                             *
*******************************************************************************

DRIVER_VERIFIER_IOMANAGER_VIOLATION (c9)
The IO manager has caught a misbehaving driver.
Arguments:
Arg1: 0000000000000228, A driver's completion routine has not marked the IRP pending if the
	PendingReturned field was set in the IRP passed to it. This may cause the OS
	to hang, especially if an error is returned by the stack.
Arg2: fffff88001c62f74, The address in the driver's code where the error was detected.
Arg3: fffff98004dd6d80, IRP address.
Arg4: 0000000000000000

Debugging Details:
------------------


BUGCHECK_STR:  0xc9_228

DRIVER_VERIFIER_IO_VIOLATION_TYPE:  228

FAULTING_IP: 
PxHlpa64+2f74
fffff880`01c62f74 49894818        mov     qword ptr [r8+18h],rcx

FOLLOWUP_IP: 
PxHlpa64+2f74
fffff880`01c62f74 49894818        mov     qword ptr [r8+18h],rcx

IRP_ADDRESS:  fffff98004dd6d80

DEVICE_OBJECT: fffffa8009699030

CUSTOMER_CRASH_COUNT:  1

DEFAULT_BUCKET_ID:  WIN7_DRIVER_FAULT Yikes?? :S

PROCESS_NAME:  System

CURRENT_IRQL:  2

LAST_CONTROL_TRANSFER:  from fffff800039593dc to fffff800034cac40

STACK_TEXT:  
fffff880`03db5228 fffff800`039593dc : 00000000`000000c9 00000000`00000228 fffff880`01c62f74 fffff980`04dd6d80 : nt!KeBugCheckEx
fffff880`03db5230 fffff800`0396347a : fffff800`039579f0 fffff880`01c62f74 fffff980`04dd6d80 00000000`00000000 : nt!VerifierBugCheckIfAppropriate+0x3c
fffff880`03db5270 fffff800`03964483 : 00000000`00000228 fffff880`01c62f74 fffff980`04dd6d80 00000000`ffffffff : nt!ViErrorFinishReport+0xda
fffff880`03db52c0 fffff800`03964654 : fffff880`01c62f74 fffffa80`09dcb810 fffff880`01c62f74 fffff980`04dd6d80 : nt!VfErrorReport1+0x63
fffff880`03db5360 fffff800`039775f9 : fffff980`04dd6ee0 00000000`00000000 fffff880`03db5530 fffff980`04dd6d80 : nt!IovpCompleteRequest4+0x64
fffff880`03db5390 fffff800`034ce6d1 : fffff980`04dd6ee3 00000000`00000000 00000000`00000000 fffff800`0358e7ab : nt!IovpLocalCompletionRoutine+0x189
fffff880`03db53f0 fffff800`0396f19f : fffff980`04dd6d80 fffff880`01faa100 fffffa80`070ec900 00000000`00000000 : nt!IopfCompleteRequest+0x341
fffff880`03db54e0 fffff880`01f994c9 : fffff980`04dd6e98 fffff800`0395914e 00000000`00000000 fffffa80`070ec901 : nt!IovCompleteRequest+0x19f
fffff880`03db55b0 fffff880`01f9ecba : fffffa80`00000000 fffff880`01faa110 00000000`00000000 fffffa80`070ec901 : storport!RaidCompleteRequestEx+0x19
fffff880`03db55e0 fffff880`01f90833 : fffffa80`070ecb10 fffff980`04dd6d80 fffffa80`09635150 fffff800`03974c42 : storport!RaUnitClaimDeviceSrb+0x7a
fffff880`03db5630 fffff880`01f8c71d : fffff880`03db57a8 fffff880`01faa110 fffffa80`098765c8 fffffa80`09699030 : storport! ?? ::FNODOBFM::`string'+0x1fd5
fffff880`03db56f0 fffff800`03975c16 : fffff980`04dd6d80 00000000`00000002 fffffa80`070ec9c0 fffff800`0397137e : storport!RaDriverScsiIrp+0x5d
fffff880`03db5730 fffff800`03974c42 : fffff980`04dd6e98 00000000`00000002 fffffa80`09876190 fffffa80`072d1f40 : nt!IovCallDriver+0x566
fffff880`03db5790 fffff800`03975c16 : fffff980`04dd6d80 00000000`00000002 fffffa80`09876040 fffffa80`09635240 : nt!ViFilterDispatchGeneric+0x62
fffff880`03db57c0 fffff880`01c634d5 : fffff980`04dd6ee0 fffffa80`09699180 fffff980`04dd6d80 fffffa80`07361ae0 : nt!IovCallDriver+0x566
fffff880`03db5820 fffff980`04dd6ee0 : fffffa80`09699180 fffff980`04dd6d80 fffffa80`07361ae0 fffffa80`00000000 : PxHlpa64+0x34d5
fffff880`03db5828 fffffa80`09699180 : fffff980`04dd6d80 fffffa80`07361ae0 fffffa80`00000000 fffffa80`00000001 : 0xfffff980`04dd6ee0
fffff880`03db5830 fffff980`04dd6d80 : fffffa80`07361ae0 fffffa80`00000000 fffffa80`00000001 fffffa80`00000000 : 0xfffffa80`09699180
fffff880`03db5838 fffffa80`07361ae0 : fffffa80`00000000 fffffa80`00000001 fffffa80`00000000 00000000`00000000 : 0xfffff980`04dd6d80
fffff880`03db5840 fffffa80`00000000 : fffffa80`00000001 fffffa80`00000000 00000000`00000000 fffffa80`098765c8 : 0xfffffa80`07361ae0
fffff880`03db5848 fffffa80`00000001 : fffffa80`00000000 00000000`00000000 fffffa80`098765c8 00000000`00000000 : 0xfffffa80`00000000
fffff880`03db5850 fffffa80`00000000 : 00000000`00000000 fffffa80`098765c8 00000000`00000000 00000000`00000000 : 0xfffffa80`00000001
fffff880`03db5858 00000000`00000000 : fffffa80`098765c8 00000000`00000000 00000000`00000000 fffffa80`0733e840 : 0xfffffa80`00000000


STACK_COMMAND:  .bugcheck ; kb

SYMBOL_NAME:  PxHlpa64+2f74

FOLLOWUP_NAME:  MachineOwner

MODULE_NAME: PxHlpa64

IMAGE_NAME:  PxHlpa64.sys

DEBUG_FLR_IMAGE_TIMESTAMP:  4a416253

FAILURE_BUCKET_ID:  X64_0xc9_228_PxHlpa64+2f74

BUCKET_ID:  X64_0xc9_228_PxHlpa64+2f74

Followup: MachineOwner
---------

0: kd> lmvm PxHlpa64
start             end                 module name
fffff880`01c60000 fffff880`01c6be00   PxHlpa64 T (no symbols)           
    Loaded symbol image file: PxHlpa64.sys
    Image path: PxHlpa64.sys
    Image name: PxHlpa64.sys
    Timestamp:        Wed Jun 24 05:16:35 2009 (4A416253)
    CheckSum:         000111B7
    ImageSize:        0000BE00
    Translations:     0000.04b0 0000.04e4 0409.04b0 0409.04e4