If you bought on Steam, I don't think you should need a product key.
If you bought on the web store, go into your account and look at your purchase history. Your key will be there, against the product. That page is a bit difficult to read if you've bought a few things, as it only shows a code and not the name of the products or images, so it's a bit hard to know what's what. If XP is all you bought though, it'll be easy
Note - your web store account and your forum account are not necessarily the same. The two systems are not linked.