LRESULT CALLBACK TimeProcedure (HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    HWND hStart, hZeroTimer;
    static HWND edTimer1;
    static float time = 0.0f;
    static bool fTimer = false;
    static UINT Timer;
    char temp[10];

    switch (message)                  /* handle the messages */
    {
        case WM_CREATE:
        {
            hStart = CreateWindow ("button", "Start / Stopp", WS_CHILD | WS_VISIBLE,
                                       60, 170, 120, 50, hWnd, (HMENU) BU_STARTTIME, 
                                       ((LPCREATESTRUCT) lParam) ->hInstance, NULL);

            hZeroTimer = CreateWindow ("button", "Nullstellung", WS_CHILD | WS_VISIBLE,
                                       215, 170, 120, 50, hWnd, (HMENU) BU_ZEROTIME, 
                                       ((LPCREATESTRUCT) lParam) ->hInstance, NULL);

            edTimer1 = CreateWindow ("edit", "", WS_CHILD | WS_VISIBLE | ES_RIGHT,
                                       60, 30, 120, 50, hWnd, NULL, 
                                       ((LPCREATESTRUCT) lParam) ->hInstance, NULL);

            break;
        }
        case WM_COMMAND:
        {
            switch (LOWORD(wParam))
            {
                case BU_ZEROTIME:
                {
                    break;
                }
                case BU_STARTTIME:
                {
                    if (fTimer)
                    {
                        KillTimer (hWnd, Timer);
                        time = 0.0f;
                        fTimer = false;
                        SendMessage (hWnd, WM_TIMER, 0, 0);
                    }
                    else
                    {
                        SetTimer (hWnd, 1, 100, NULL);
                        fTimer = true;
                    }
                    break;
                }
                default:
                    break;
            }
            break;
        }
        case WM_TIMER:
        {
            time += 0.1f;
            sprintf (temp, "%.2f", time);
            SetWindowText (edTimer1, temp);
            UpdateWindow (edTimer1);
            break;
        }
        case WM_DESTROY:
            PostQuitMessage (0);       
            break;
        default:                      
            return DefWindowProc (hWnd, message, wParam, lParam);
    }
    return 0;
}

Hab schon viel rumprobiert, bin aber auf keine Lösung gestoßen.

WNDCLASSEX wincl;

            wincl.hInstance = ((LPCREATESTRUCT) lParam) ->hInstance;
            wincl.lpszClassName = szTimeName;
            wincl.lpfnWndProc = TimeProcedure;
            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 = CreateSolidBrush (RGB (128, 128, 128));

            RegisterClassEx (&wincl);

            lTime = CreateWindow (szTimeName, "", WS_CHILD, 
                                   400, 305, 390, 235, hWnd, NULL, 
                                   ((LPCREATESTRUCT) lParam) ->hInstance, NULL);

Und das tolle daran ist, das man den Fehler dann auch meistens selbst findet, wenn man den Code so weit eingrenzen kann. Nimm ALLES raus was nichts mit dem Fehler zu tun hat.