|Could be your drive or the CD itself. If the CD is dirty, try washing it and gently drying. Try copying the CD contents to your hard drive and then run the setup from there. |
If none of the above works and you know someone that owns that same version of Office, you can copy their CD and use your installation key. That is perfectly legal.
In the future I suggest you make copies of any new software and then install from the copy. Put the original away for safe keeping. That is also legal.
If possible you could also try installing to another computer to verify that the CD is OK. If so, then your optical drive may be failing or the laser lens is dirty. Lens cleaning kits are available.