你可以这样来实现：（支持Win XP下的关机）
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack=1)]
   internal struct TokPriv1Luid
  {
   public int Count;
   public long Luid;
   public int Attr;
  }
  

  [DllImport("kernel32.dll", ExactSpelling=true) ]
  internal static extern IntPtr GetCurrentProcess();

  [DllImport("advapi32.dll", ExactSpelling=true, SetLastError=true) ]
  internal static extern bool OpenProcessToken( IntPtr h, int acc, ref IntPtr phtok );

  [DllImport("advapi32.dll", SetLastError=true) ]
  internal static extern bool LookupPrivilegeValue( string host, string name, ref long pluid );

  [DllImport("advapi32.dll", ExactSpelling=true, SetLastError=true) ]
  internal static extern bool AdjustTokenPrivileges( IntPtr htok, bool disall,
   ref TokPriv1Luid newst, int len, IntPtr prev, IntPtr relen );

  [DllImport("user32.dll", ExactSpelling=true, SetLastError=true) ]
  internal static extern bool ExitWindowsEx( int flg, int rea );

  internal const int SE_PRIVILEGE_ENABLED = 0x00000002;
  internal const int TOKEN_QUERY = 0x00000008;
  internal const int TOKEN_ADJUST_PRIVILEGES = 0x00000020;
  internal const string SE_SHUTDOWN_NAME = "SeShutdownPrivilege";
  internal const int EWX_LOGOFF = 0x00000000;
  internal const int EWX_SHUTDOWN = 0x00000001;
  internal const int EWX_REBOOT = 0x00000002;
  internal const int EWX_FORCE = 0x00000004;
  internal const int EWX_POWEROFF = 0x00000008;
  internal const int EWX_FORCEIFHUNG = 0x00000010;

  private void DoExitWin( int flg )
  {
   bool ok;
   TokPriv1Luid tp;
   IntPtr hproc = GetCurrentProcess();
   IntPtr htok = IntPtr.Zero;
   ok = OpenProcessToken( hproc, TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, ref htok );
   tp.Count = 1;
   tp.Luid = 0;
   tp.Attr = SE_PRIVILEGE_ENABLED;
   ok = LookupPrivilegeValue( null, SE_SHUTDOWN_NAME, ref tp.Luid );
   ok = AdjustTokenPrivileges( htok, false, ref tp, 0, IntPtr.Zero, IntPtr.Zero );
   ok = ExitWindowsEx( flg, 0 );
  }

  public void Reboot()
  {
   DoExitWin( EWX_REBOOT );
  }

                  //点击按钮，执行关机事件
  private void button1_Click(object sender, System.EventArgs e)
  {
   DoExitWin(1);
  }