Stop A is frequently a driver problem.
In this case the driver appears to be your CPU driver and the box bugchecked while the idle process was running. You can see if there are driver updates for amdk8.sys from AMD and also see if there are BIOS updates available for your system (which would include microkernel updates). It's really looking like a bad CPU though.

Sorry
Ping dclive here on AT. His debugging kung-fu is far beyond mine.
Loading Dump File [XXomittedXX]
Mini Kernel Dump File: Only registers and stack trace are available
Symbol search path is: XXomittedXX
Executable search path is:
Windows XP Kernel Version 2600 (Service Pack 2) MP (2 procs) Free x86 compatible
Product: WinNt, suite: TerminalServer SingleUserTS
Built by: 2600.xpsp_sp2_qfe.070227-2300
Kernel base = 0x804d7000 PsLoadedModuleList = 0x8055c700
Debug session time: Tue May 1 20:41:39.906 2007 (GMT-4)
System Uptime: 0 days 6:40:54.610
Loading Kernel Symbols
...................................................................................................................................................
Loading User Symbols
Loading unloaded module list
....................
*******************************************************************************
* *
* Bugcheck Analysis *
* *
*******************************************************************************
Use !analyze -v to get detailed debugging information.
BugCheck 1000000A, {0, ff, 0, 806e2ea0}
Unable to load image AmdK8.sys, Win32 error 2
*** WARNING: Unable to verify timestamp for AmdK8.sys
*** ERROR: Module load completed but symbols could not be loaded for AmdK8.sys
Probably caused by : hardware ( AmdK8+3552 )
Followup: MachineOwner
---------
0: kd> !analyze -v
*******************************************************************************
* *
* Bugcheck Analysis *
* *
*******************************************************************************
IRQL_NOT_LESS_OR_EQUAL (a)
An attempt was made to access a pageable (or completely invalid) address at an
interrupt request level (IRQL) that is too high. This is usually
caused by drivers using improper addresses.
If a kernel debugger is available get the stack backtrace.
Arguments:
Arg1: 00000000, memory referenced
Arg2: 000000ff, IRQL
Arg3: 00000000, value 0 = read operation, 1 = write operation
Arg4: 806e2ea0, address which referenced memory
Debugging Details:
------------------
READ_ADDRESS: 00000000
CURRENT_IRQL: ff
FAULTING_IP:
hal!HalpClockInterrupt+4c
806e2ea0 8b1d00000000 mov ebx,dword ptr ds:[0]
CUSTOMER_CRASH_COUNT: 1
DEFAULT_BUCKET_ID: DRIVER_FAULT
BUGCHECK_STR: 0xA
PROCESS_NAME: Idle
MISALIGNED_IP:
hal!HalpClockInterrupt+4c
806e2ea0 8b1d00000000 mov ebx,dword ptr ds:[0]
UNALIGNED_STACK_POINTER: 805503c1
LAST_CONTROL_TRANSFER: from b95ee552 to 806e2ea0
STACK_TEXT:
805503c0 b95ee552 badb0d00 00000014 8055040c hal!HalpClockInterrupt+0x4c
WARNING: Stack unwind information not available. Following frames may be wrong.
80550430 ffdffc70 80527b3b ffdff000 8055b0a0 AmdK8+0x3552
80550450 805450cc 00000000 0000000e 00000000 0xffdffc70
80550454 00000000 0000000e 00000000 00000000 nt!KiIdleLoop+0x10 [XXomittedXX]
STACK_COMMAND: kb
FOLLOWUP_IP:
AmdK8+3552
b95ee552 ?? ???
SYMBOL_STACK_INDEX: 1
FOLLOWUP_NAME: MachineOwner
IMAGE_NAME: hardware
DEBUG_FLR_IMAGE_TIMESTAMP: 0
SYMBOL_NAME: AmdK8+3552
MODULE_NAME: hardware
FAILURE_BUCKET_ID: IP_MISALIGNED
BUCKET_ID: IP_MISALIGNED
Followup: MachineOwner
---------