That's how the trial version works. It lets you try the product
for a period of time, and when that period of time expires, you
have to buy the software to continue to use it. See
http://support.microsoft.com/?kbid=831023 "How to convert an
Office 2003 Trial edition to a full retail edition" for more
information.
If you had an older version of Word or Office installed on your
computer before downloading the Office 2003 trial, and if you
chose to remove the previous version when you installed the
Office 2003 trial, then you can reinstall the older version from
the original CD.
