а еще лучше вставь код сюда, который у тебя сейчас.

#include <windows.h>

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

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
{
static wchar_t szAppName[] = L"HelloWin";
HWND hwnd;
MSG msg;
WNDCLASSEX wndclass;

wndclass.cbSize = sizeof(wndclass);
wndclass.style = CS_HREDRAW | CS_VREDRAW;
wndclass.lpfnWndProc = WndProc;
wndclass.cbClsExtra = 0;
wndclass.cbWndExtra = 0;
wndclass.hInstance = hInstance;
wndclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wndclass.hCursor = LoadCursor(NULL, IDC_ARROW);
wndclass.hbrBackground =(HBRUSH) GetStockObject(WHITE_BRUSH);
wndclass.lpszMenuName = NULL;
wndclass.lpszClassName = szAppName;
wndclass.hIconSm = LoadIcon(NULL, IDI_APPLICATION);

RegisterClassEx(&wndclass);
return 0;
}

If you are using Visual Studio 2005/2008 Standard Edition, no configuration is needed to build
Intel® 64 architecture applications. For other editions:
1. From Control Panel > Add or Remove Programs, select “Microsoft Visual Studio 2005”
(or 2008) > Change/Remove. The Visual Studio Maintenance Mode window will appear.
Click Next.
2. Click Add or Remove Features
3. Under “Select features to install”, expand Language Tools > Visual C++
4. If the box “X64 Compiler and Tools” is not checked, check it, then click Update. If the
box is already checked, click Cancel.
To use Microsoft Visual Studio 2005/2008 Team System Edition to build applications to run on
IA-64 architecture systems, follow the above steps and ensure that the box “Itanium Compiler
and Tools” is checked.

и еще вопрос:
ничего что я вин апи учу по книге Ч. Петзольд "Программирование для Windows 95
Она просто по вин апи для С а не для С++