If Word 2007 came pre-installed on a computer you bought (such as a Dell or HP laptop), the product key is usually on a "Certificate of Authenticity" (COA) sticker. This sticker is often found on the bottom of a laptop, the back of a desktop tower, or sometimes under the battery compartment.
If Word 2007 came with your computer, the key should be in the documentation provided by the manufacturer. How to Recover a Lost Key from Your PC product key for microsoft word 2007
If Word 2007 is already installed but you lost the physical key, you can extract it from your system registry using specialized tools: Office 2007 Serial key recovery? If Word 2007 came pre-installed on a computer