On a retail PC, the product key is usually printed on a Certificate of Authenticity label. It's usually stuck to the computer (base unit) somewhere on the top of the side, and on laptops it's usually stuck on the underside.
On rare occasions, it may be stuck to the casing of the system restore disc (if one was provided).
If you can boot up into the system in question, and it's already been activated with a product key, you can use a freeware program like NirSoft ProduKey
or Magical Jelly Bean Keyfinder
to find out what it is.