If this CD is bootable (you test it on other PCs) then try duplicating the CD and trying the duplicate.
Other ideas include.
1. Cleaning the CD/DVD lens.
2. Replacing the old drive.
I have an old IBM Aptiva model 2174-595 and I can't get it to boot from CD. What I want to do is reformat and reinstall a fresh copy of XP but I need to boot from CD to do that. I have checked BIOS and it is set to boot from optical drive first and 1st IDE HD second but it always just goes to the old Windows startup. I have disconnected and reconnected all of the cables. I know the DVD drive works since it runs fine after Windows starts. I know the IBM is old but I'm just using it in my garage.