Was this a casual purchase, i.e. from an individual rather than a company. If you don`t have a company invoice with Office 2003 specifically listed on it there`s little you can do. It could well be a "cracked copy" and therefore illegal. I would have thought you should have been given the original installation disks or the Certificate of Authenticity when you bought the computer if it were genuine.
If you want to make it legitimate contact Microsoft and ask them what to do. They will offer to send you a full CD and registration key but remember that "IF" you are a student or teacher there is a version of Office 2003 that costs no more than £120.
Another alternative is to uninstall Office and use the free open source OpenOffice click here although it doesn`t have the equivalent of Outlook it does have compatible programs for Word, Excel, Powerpoint and Access - the latter not available in the Office Student & Teacher edition. OpenOffice will open and save files in Officeformats such as .doc & .xls