If you have an expired trial version of Office 2003, then you can either
purchase a full version at your favorite retailer, or you can purchase a
product key online from Microsoft that you can use to convert your trial
version to a full version. If you don't have the Office 2003 CD, it
would probably be better if you purchased a retail copy, because then
you will have the CD in case you need to reinstall or update Office.
See http://www.microsoft.com/office/trial/convert.mspx for more information.
If you are talking about some other version of Office, write back with
MUCH more information.