Another computer problem

Everytime I try to start Microsoft Excel, a box comes up saying Preparing Files. After a few seconds another box comes up reading:
Installation Error: Files not Found. A required installation file SKU011.CAB could not be found
Original Installation Source Required: If you installed Microsoft Office Professional Edition 2003 from a CD, please insert your CD. If you installed Microsoft Office Professional Edition 2003 over your computer network, please browse to the installation source on your network. Once you have located your installation source, Click OK.
The problem is that I can't find my CD (must've been at least a few years since I installed Microsoft 2003, and I don't know what it means by "over your computer network". Can anyone help me here? Thanks.