Microsoft Office 2003 (XP) is an old product, and most likely was discontinued by Microsoft since newer versions were released (Office 2013). In this case you could download a trial version of the software using Google and then use your key but it's not recommended as downloads from 3rd party resources could be infected with malware.
As an alternative way you may contact Microsoft and explain the situation and I believe they will provide ways to get your Office 2003 back.