I hope it ok to post that here.
Sure. Buying a product key from MS isn't illegal.
As to product keys in general - brand name computers usually come with the product key attached on a holographic sticker. On desktops, it's usually on the side, and on laptops, it's usually on the bottom.
Those with restore disks or restore partitions usually don't need a product key because it's automatically installed as part of the installation or re-installation.
However, if you have a version of windows installed with either an OEM or a full version of the software from the disk, the sticker is on the shrink wrap the disk came in, and should have been applied to the case. You cannot use the same product key for more than one computer. It must be activated within thirty days. If you use the same product key on two or more computers, they will all be shut off.
The software, as I understand it, is tied to the motherboard. Once the activation process is complete, it has taken inventory of all the hardware on the computer. This includes such things as serial number on the motherboard. Microsoft considers the motherboard to be the computer and if you replace the motherboard, they usually want you to replace the operating system as well.
If the software has been activated in the past and is still on the computer, then the product key should be available with a program like Magical Jellybean Keyfinder. If the software has been re-installed and is asking for a product key, there is no (legal) way around it.
But by the time you need to activate Windows, you should already have the product key installed because it is entered during the install process, not during the activation process. If Windows is up and running and you don't know your product key, then you can use the Keyfinder to get the key. Write it down and keep it in a safe place. But you can activate over the internet automatically (assuming you have an active internet connection). To do this, you should not need to know the product key. You will need it if you need to re-install Windows XP from a CD that is not a Dell, HP, Compaq or similar brand name.