If you have a valid copy of Office, Xp, 2003, 2007, or 2010; however you are getting an alert stating it is not valid. This Article from Microsoft explains what may be the cause.
http://support.microsoft.com/kb/903275/en-us
In some cases the File validating office may become corrupt, and you can follow these directions or run the "fix it for me" tool and it will delete the corrupted file, you will then need to reactivate office and re validate.