This may be a much simpler approach for XP and up...

Code:
#include <windows.h>
#include <stdio.h>
#include <tchar.h>

int main(void)
{
    typedef BOOL (WINAPI *LPFNIUA)(void);

    HMODULE hShell32 = LoadLibrary(_T("shell32.dll"));
    LPFNIUA pIsAdmin = NULL;

    if (hShell32)
    {
        pIsAdmin = (LPFNIUA)GetProcAddress(hShell32, MAKEINTRESOURCE(680));
        if (pIsAdmin)
        {
            if(pIsAdmin())
                printf(_T("User is Admin\n"));
            else printf(_T("User is NOT Admin\n"));
        }
        FreeLibrary(hShell32);
    }
    return 0;
}