Visual C++ 5 error message....I can't remember what it means!

I've been trying to compile this source code from a Windows program that does absolutely nothing at this point:

#include <windows.h>

LRESULT CALLBACK wndProc(HWND w,UINT msg,WPARAM wp,LPARAM lp)
{
  switch(msg) {
  case WM_CREATE:
     return 0;
  case WM_RBUTTONDOWN:
  case WM_CLOSE:
     PostQuitMessage(0);
     DestroyWindow(w);
     return 0;
  }
  return DefWindowProc(w,msg,wp,lp);
}

int WINAPI WinMain(
 HINSTANCE hInstance,  // handle to current instance
 HINSTANCE hPrevInstance,  // handle to previous instance
 LPSTR lpCmdLine,      // pointer to command line
 int nCmdShow          // show state of window
)
{
  WNDCLASS cls;
  HWND w;
  MSG msg;

  memset(&cls,0,sizeof(cls));
  cls.lpfnWndProc=wndProc;
  cls.hInstance=GetModuleHandle(0);
  cls.lpszClassName="class";
  cls.hbrBackground=(HBRUSH)(COLOR_WINDOW+1);
  RegisterClass(&cls);
  w=CreateWindowEx(0,"class","test",
     WS_POPUP|WS_VISIBLE|WS_SYSMENU|WS_CAPTION,
     5,5,200,200,0,0,cls.hInstance,0);
  while(GetMessage(&msg,0,0,0)) {
     TranslateMessage(&msg);
     DispatchMessage(&msg);
  }
}

The problem is, when I try to build this in Visual C++ 5 I get these two error messages:

Linking...
LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main
Debug/Proj.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

What's wrong?  What I am doing wrong?  Thanks for any help! :D