The Telegraph had a useful post on how to find your windows product key.

If you wanted to reinstall Windows on a computer but the label on the case is too worn to able to read the product key here’s what you do.

This information is encrypted so you will need help from a free utility called Belarc Advisor. Not only will this decrypt and show your Windows Product key, and those of any other MS products you might have, it reveals a vast amount of information about your computer’s hardware and software, which it displays in a browser window.