Have you tried the Maigc Jellybean Keyfinder?

Or this:

"If you have Vista all you need to do is go:
Control panel/system maitenance/
Find which version of Windows you are using/
Show more details

And there you will find your Product ID (or Key). "

I used the same product key that I installed Vista with.. It's a no-go...

Please enter the 25-character product key that came with your purchase for validation. You may include the dashes when entering the Product Key, but the dashes are not necessary. You’ll find the product key on a yellow sticker on the back of your DVD media holder. Please keep this product key! You will need it to install and activate Windows Vista when you receive your alternate media.