bool __fastcall IsAdminUser( void )
{
    typedef HRESULT (WINAPI *ISUSERANADMIN)(void);

        ISUSERANADMIN   IsUserAnAdmin;
        HANDLE  hShell32;
        bool    rc = false;

    hShell32 = LoadLibrary( TEXT("SHELL32.DLL") );
    if( hShell32 ){
        IsUserAnAdmin = (ISUSERANADMIN)
                GetProcAddress((HINSTANCE)hShell32, (LPCSTR)680);
        rc = IsUserAnAdmin();
        FreeLibrary((HINSTANCE)hShell32);
    }
    return( rc );
}

<< 使用例 >>
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    if( IsAdminUser() ) ShowMessage("管理者です");
}