To recover your lost the product key of Windows XP or most versions of Office and have lost the CD case you can use ProduKey.
ProduKey is a small utility that displays the ProductID and the CD-Key of MS-Office, Windows, Exchange Server, and SQL Server installed on your computer. You can view this information for your current running operating system, or for another operating system/computer – by using command-line options. This utility can be useful if you lost the product key of your Windows/Office, and you want to reinstall it on your computer.