If you have a legitimate installation of Microsoft Office Professional 2003 and need a replacement disk, you can try calling the support number here: http://support.microsoft.com/kb/302822
Press 0 to get an operator and they will help you.
Note: If you have an OEM version installed then you'll need to contact your original dealer. The way you can tell if you have an OEM version installed is by going to the Help > About and checking the product ID. If there are any letters in the ID then you have an OEM version.