Google Checkout Callback URL

How do I make this work with discountasp.net?

I read the article that said to put the callback url in a seperate folder and turn off basic authentication to that folder and to check the merchant id/key programatically

I guess what I'm asking for is a code sample in ASP or VBscript that will read the http auth headers, perform base64 decode and validate the merchant ID and merchant key.....

help