An aging CD/DVD drive. Along with other signs, this is one of the others that the drive is aging. Try a new CD/DVD drive. Or at least another copy of the CD, a lens cleaner.
I have used the CD to reinstall the OS at least 10-15 times without problem. However, now, all of a sudden, it refuses to boot. I have a backup copy (which was made early on when it used to boot), which also refuses to boot! I know how to make the PC boot from the CD, and all that.
Can I copy the OS files to my hard drive and make an ISO copy? Or, do you have any better ideas?