What is the product key of windows XP? - Microsoft Windows XP Professional for PC

Each key is unique.
It is contained on a sticker that should be attached to your comptuer. Laptops are found on the bottom, and towers usually along the side, but have been found on the bottoms, and tops.

You can find your installed Windows key using many free tools available to you.

I like the one from Piriform called SPECCY.
After you install it you can see your XP Key.

NOTE: Don't install the yahoo toolbar included with Speccy.
Just uncheck the box.

If you are just installing XP from a NEW holographic xp disc then you should have an XP Key Sticker on the package that the disc came in.
You need to remove the sticker and place it on your computer somewhere convenient.