What OS (I know it's windows but which one) are you using and what version of PHP. I have Linux and Windows PHP set up and I did not get that message using the same logic on either platform.
Consequently, I'm unfamiliar with the meaning of the "Notice: Undefined index:" message.
You can use isset($ppnews_id) by somebody could just append a ?ppnews_id=anaything and go right through without logging in.
Anyway, you may have an older version of PHP (<4.2.0) that doesn't recognize the superglobal arrays.