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Fundamentally, if you have permissions to run shutdown.exe, you have permissions to use the same API that it uses, so you can do what you want in your own program. I would use the ExitWindowsEx() API, maybe with a warning dialog and a countdown. You need to obtain shutdown permission first though. I found this code snippet thanks to CoderSource.net :

HANDLE hToken; // handle to process token 
TOKEN_PRIVILEGES tkp; // pointer to token structure
OpenProcessToken(GetCurrentProcess(),TOKEN_ADJUST_PRIVILEGES | 
       TOKEN_QUERY, &hToken); // Get the LUID for shutdown privilege.              
LookupPrivilegeValue(NULL, SE_SHUTDOWN_NAME, &tkp.Privileges[0].Luid); 
tkp.PrivilegeCount = 1; // one privilege to set
tkp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;   
// Get shutdown privilege for this process. 
AdjustTokenPrivileges(hToken, FALSE, &tkp, 0, (PTOKEN_PRIVILEGES) NULL, 0);
// Cannot test the return value of AdjustTokenPrivileges.  
if (GetLastError() != ERROR_SUCCESS)   
   MessageBox("AdjustTokenPrivileges enable failed.");

MSDN offers this example on the use of ExitWindowsEx :

#include <windows.h>

#pragma comment(lib, "user32.lib")
#pragma comment(lib, "advapi32.lib")

BOOL MySystemShutdown()
{
   HANDLE hToken; 
   TOKEN_PRIVILEGES tkp; 
 
   // Get a token for this process. 
 
   if (!OpenProcessToken(GetCurrentProcess(), 
        TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken)) 
      return( FALSE ); 
 
   // Get the LUID for the shutdown privilege. 
 
   LookupPrivilegeValue(NULL, SE_SHUTDOWN_NAME, 
        &tkp.Privileges[0].Luid); 
 
   tkp.PrivilegeCount = 1;  // one privilege to set    
   tkp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED; 
 
   // Get the shutdown privilege for this process. 
 
   AdjustTokenPrivileges(hToken, FALSE, &tkp, 0, 
        (PTOKEN_PRIVILEGES)NULL, 0); 
 
   if (GetLastError() != ERROR_SUCCESS) 
      return FALSE; 
 
   // Shut down the system and force all applications to close. 
 
   if (!ExitWindowsEx(EWX_SHUTDOWN | EWX_FORCE, 
               SHTDN_REASON_MAJOR_OPERATINGSYSTEM |
               SHTDN_REASON_MINOR_UPGRADE |
               SHTDN_REASON_FLAG_PLANNED)) 
      return FALSE; 

   //shutdown was successful
   return TRUE;
}

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