i hope this works
Code:
#include <windows.h>

#define ID_LIST 1
#define ID_TEXT 2
HINSTANCE hChildInst = 0;


LRESULT CALLBACK WindowProcedure (HWND, UINT, WPARAM, LPARAM);


char szClassName[ ] = "ComboBox App";

int WINAPI WinMain (HINSTANCE hThisInstance,
                    HINSTANCE hPrevInstance,
                    LPSTR lpszArgument,
                    int nFunsterStil)

{
    HWND hwnd;              
    MSG messages;            
    WNDCLASSEX wincl;       
	WNDCLASSEX wincl2;

    
    wincl.hInstance = hThisInstance;
    wincl.lpszClassName = szClassName;
    wincl.lpfnWndProc = WindowProcedure;      
    wincl.style = CS_DBLCLKS;                 
    wincl.cbSize = sizeof (WNDCLASSEX);

   
    wincl.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
    wincl.hCursor = LoadCursor (NULL, IDC_ARROW);
    wincl.lpszMenuName = NULL;                
    wincl.cbClsExtra = 0;                     
    wincl.cbWndExtra = 0;                     
  
    wincl.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

  
    if (!RegisterClassEx (&wincl))
        return 0;


	/* The Window structure */
    wincl2.hInstance = hChildInst;
    wincl2.lpszClassName = "Proc2";
    wincl2.lpfnWndProc = WindowProcedure;     
    wincl2.style = CS_DBLCLKS;               
    wincl2.cbSize = sizeof (WNDCLASSEX);

    /* Use default icon and mouse-pointer */
    wincl2.hIcon = LoadIcon (NULL, IDI_APPLICATION);
    wincl2.hIconSm = LoadIcon (NULL, IDI_APPLICATION);
    wincl2.hCursor = LoadCursor (NULL, IDC_ARROW);
    wincl2.lpszMenuName = NULL;                
    wincl2.cbClsExtra = 0;                   
    wincl2.cbWndExtra = 0;                   
 
    wincl2.hbrBackground = (HBRUSH) COLOR_BACKGROUND;

 
    if (!RegisterClassEx (&wincl2))
        return 0;


 
    hwnd = CreateWindowEx (
           0,                  
           szClassName,         
           "ComboBox App",       
           WS_OVERLAPPEDWINDOW, 
           CW_USEDEFAULT,       
           CW_USEDEFAULT,       
           544,                 
           375,                 
           HWND_DESKTOP,       
           NULL,               
           hThisInstance,       
           NULL                
           );

    
    ShowWindow (hwnd, nFunsterStil);

    
    while (GetMessage (&messages, NULL, 0, 0))
    {
       
        TranslateMessage(&messages);
        
        DispatchMessage(&messages);
    }

    
    return messages.wParam;
}



void FillListBox(HWND hwndList)
{
	CHAR *pVarName[] = {"Item1", "Item2", "Item3", "Item4", "Item5"};
     
     for(int i=0; i<5; i++)
  
             SendMessage(hwndList, CB_ADDSTRING, 0, (LPARAM)pVarName[i]);
     
}
             
HWND hwndtest;


LRESULT CALLBACK WindowProcedure (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static HWND hwndList, hwndText, hwndComboWindow;
    int iIndex, iLength, cxChar, cyChar;
    CHAR pVarName[30];
        
     
        
    switch (message)                 
    {
    
        case WM_CREATE:
             
             cxChar = LOWORD(GetDialogBaseUnits());
             cyChar = HIWORD(GetDialogBaseUnits());
             
           //Create Display Window
           hwndText = CreateWindow(TEXT("static"),NULL, WS_CHILD | WS_VISIBLE |
                                   SS_LEFT, cxChar, cyChar, 
                                   GetSystemMetrics(SM_CXSCREEN), cyChar, 
                                   hwnd, (HMENU)ID_TEXT, 
                                   (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE), 
                                   NULL);
             
           //Create ComboBox
           hwndList =  CreateWindow(TEXT("Combobox"), NULL, WS_CHILD | WS_VISIBLE |
                                   LBS_STANDARD, cxChar, cyChar*3, 
                                   cxChar*16 + GetSystemMetrics(SM_CXVSCROLL), 
                                   cyChar*5, hwnd, (HMENU)ID_LIST, 
                                   (HINSTANCE)GetWindowLong(hwnd, GWL_HINSTANCE),
                                   NULL);

								   
		
                                       
             //Populate ComboBox
             FillListBox(hwndList);
             
             //Set a Default Selection
             SendMessage(hwndList, CB_SETCURSEL, 0, 0);
             GetWindowText(hwndList, pVarName, 30);
             SetWindowText(hwndText, pVarName);
             
             return 0;
             
        case WM_COMMAND:
             
             if( LOWORD(wParam)==ID_LIST && HIWORD(wParam)==CBN_SELCHANGE )
             {
                        
                 GetWindowText(hwndList, pVarName, 30);      
                 
               
                 SetWindowText(hwndText, pVarName);
			
				 
     
             
				 if( strcmp(pVarName,"Item3") == 0 ){
				
					
				CreateWindowEx( WS_EX_CLIENTEDGE,
					"Proc2",
					(LPCSTR) pVarName,
					WS_OVERLAPPEDWINDOW | WS_POPUP | WS_VISIBLE | WS_CLIPSIBLINGS,  
					760,       
					0,        
					400,       
					440,        
					0,
					NULL,
					hChildInst,
					NULL );
				
					
					if( hwnd == NULL ){
			MessageBox( NULL, "Create Window failed", "Error", MB_OK );
			}
			
			//	UpdateWindow( hwndComboWindow );	

					}

				 if( strcmp(pVarName,"Item4") == 0 ){
				
					
				CreateWindowEx( WS_EX_CLIENTEDGE,
					"Proc2",
					(LPCSTR) pVarName,
					WS_OVERLAPPEDWINDOW | WS_POPUP | WS_VISIBLE | WS_CLIPSIBLINGS,  
					760,       
					0,       
					400,      
					440,      
					hwnd,
					NULL,
					hChildInst,
					NULL );
				
					
					if( hwnd == NULL ){
			MessageBox( NULL, "Create Window failed", "Error", MB_OK );
			}
			
		//		UpdateWindow( hwndComboWindow );	

					}
		
			 }
				
				break;  
			
             return 0;    
           
        case WM_DESTROY:
             
            PostQuitMessage (0);      
            break;
        
        default: 
                                    
            return DefWindowProc (hwnd, message, wParam, lParam);
    }

    return 0;
}