WinDbg U command failed.

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Hello,everybody. recently used the WinDbg Debugger module,but i sometimes to use the U command will fail,Google to answer seems is set load the symbol of the problem,and then I set symbol path,but still failed,I startup WinDbg after choice the Kernel Debug,this is my WinDbg display of the information,I hope someone can help me analyze problem. thank you in advance!

Microsoft (R) Windows Debugger Version 6.12.0002.633 X86
Copyright (c) Microsoft Corporation. All rights reserved.

Connected to Windows XP 2600 x86 compatible target at (Tue May  1 22:45:36.156 2012 (UTC + 8:00)), ptr64 FALSE
Symbol search path is: *** Invalid ***
****************************************************************************
* Symbol loading may be unreliable without a symbol search path.           *
* Use .symfix to have the debugger choose a symbol path.                   *
* After setting your symbol path, use .reload to refresh symbol locations. *
****************************************************************************
Executable search path is: 
*********************************************************************
* Symbols can not be loaded because symbol path is not initialized. *
*                                                                   *
* The Symbol Path can be set by:                                    *
*   using the _NT_SYMBOL_PATH environment variable.                 *
*   using the -y <symbol_path> argument when starting the debugger. *
*   using .sympath and .sympath+                                    *
*********************************************************************
*** ERROR: Symbol file could not be found.  Defaulted to export symbols for ntkrpamp.exe - 
*******************************************************************************
WARNING: Local kernel debugging requires booting with kernel
debugging support (/debug or bcdedit -debug on) to work optimally.
*******************************************************************************
Windows XP Kernel Version 2600 (Service Pack 3) MP (2 procs) Free x86 compatible
Product: WinNt, suite: TerminalServer SingleUserTS
Built by: 2600.xpsp_sp3_gdr.101209-1647
Machine Name:
Kernel base = 0x804d8000 PsLoadedModuleList = 0x8055e720
Debug session time: Tue May  1 22:45:36.187 2012 (UTC + 8:00)
System Uptime: 0 days 0:02:41.823
lkd> u NtWaitForDebugEvent
Couldn't resolve error at 'NtWaitForDebugEvent'       ########    fail
lkd> u 
00000000 ??              ???
      ^ Memory access error in 'u '
lkd> u NtWaitForDebugEvent
Couldn't resolve error at 'NtWaitForDebugEvent'       ########    fail
lkd> u 
00000001 ??              ???
      ^ Memory access error in 'u '
lkd> u NtSetInformationDebugObject
Couldn't resolve error at 'NtSetInformationDebugObject' ########    fail
lkd> u NtOpenProcess
nt!NtOpenProcess:
805cc440 68c4000000      push    0C4h
805cc445 68c0b44d80      push    offset nt!FsRtlLegalAnsiCharacterArray+0x2010 (804db4c0)
805cc44a e89107f7ff      call    nt!wctomb+0x45 (8053cbe0)
805cc44f 33f6            xor     esi,esi
805cc451 8975d4          mov     dword ptr [ebp-2Ch],esi
805cc454 33c0            xor     eax,eax
805cc456 8d7dd8          lea     edi,[ebp-28h]
805cc459 ab              stos    dword ptr es:[edi]
lkd> u NtSetInformationDebugObject
Couldn't resolve error at 'NtSetInformationDebugObject'