Some software stores the license keys in plain text in the configuration files. With software of that nature, you can just read the configuration file and see your license key.
Other software stores license keys in the Windows registry. You can just look into the registry and find your license keys.
With Microsoft Office, it is a bit more tricky. Microsoft tries to make it difficult for you to find your license keys.
However, tools such as
Magical Jelly Bean Keyfinder are designed to help you do just that.
Keyfinder supports Windows 95, 98, ME, NT4, 2000, XP, Server 2003, and Vista. Keyfinder also supports Office 97, XP, and 2003.