If you are entering the key during product installation, be sure you're typing it correctly. It is sometimes difficult to tell "8" from "B", "0" from "O", "5" from "S", etc. on the printed key sticker on the package. Also, you need to make sure the key is actually for that version of the software.
If you purchased this software and the included key doesn't work, you need to contact Microsoft to fix the issue.
This page at MS explains how to reach the Activation Center for your area and your software. This will only work if the key is legitimate, though. If you purchased a counterfeit copy of Office or you're trying to use a key from a key generator program, the keys won't validate.
MS no longer offers Office 2007, and you can't just purchase a new key for a copy you may have. All they want you to do now is purchase Office 2010. You can do that if you want to, but you can also download and install
OpenOffice instead. OO is a 100% free alternative to MS Office, with the same word processing, spreadsheet, presentation, etc. capabilities. OO can use MS formats like docx so you can work with files you've already created, as well as many others. If you don't absolutely need MS Office, OO is the way to go.