You need to update your Windows header files. That function is defined in winuser.h, so you'll need to paste its definition into the file already on your system. Here's the listing from the SDK:

Code:
WINUSERAPI UINT WINAPI GetWindowModuleFileNameA(
   IN HWND hwnd,
   OUT LPSTR pszFileName,
   IN UINT cchFileNameMax);
WINUSERAPI UINT WINAPI GetWindowModuleFileNameW(
   IN HWND hwnd,
   OUT LPWSTR pszFileName,
   IN UINT cchFileNameMax);
#ifdef UNICODE
#define GetWindowModuleFileName GetWindowModuleFileNameW
#else
#define GetWindowModuleFileName GetWindowModuleFileNameA
#endif
You might want to download a fresh edition of the SDK (November is the most current), especially if you're using Dev-C++.