Microsoft includes the Product key with the documentation that came with the install CD. But if it has been lost there is a remedy.
Product key replacement is done solely through Microsoft, but finding the right place to do it can be daunting. Here's how to cut through the red tape, most of it anyhow:
Click here to bring up Microsoft's product replacement page.
Scroll down a few lines. On the left side, under the second 'More Information' header, click on 'Replacement Product Key'.
This will walk you through the procedure. You'll be up and running in no time. Mike