Thanks for the pointers. This information may help me solve this problem.

Although I have not done some of the test myself, an IT person did some for me. The college where I teach is licensed to issue Windows XP Pro CDs to its faculty members. That's how I upgraded my Windows XP Home to Pro (at that point, my computer read boot CDs). But when SP2 messed up my ports, I decided to reinstall Windows XP Pro+SP2 (new disk) from scratch. It was at that point that the boot problem started.

I took my laptop to my office and an IT technician tested the CDs ruined by my computer on my office computer (to which I do not have administrative access). The disks were in fact coasters. He then burned a third XP Pro CD and tested it in one of his computers, which proved the CD worked. I have not iserted that CD in my computer just in case, and I am not sure at what speed he burns CDs.

Now, he claims the third XP Pro CD he burned worked fine, and I took his claims in good faith (but with a pinch of skepticism, I must admit).
So in a way the boot CDs that don't run in my computer run well on a different computer, but I don't have first-hand experience of that.

I do not understand what the CD drive does to a CD that has been written on and closed. In theory, a closed CD is no longer R but only ROM. If I can't solve this problem, I'll have to take my computer to a service center, but that'll be the last resort.

As it is, I'm looking forward to running Linux, and the chance to run it from a CD sounds fantastic. I'm going to try the boot floppy-to-CD route first. Since I still have the Knoppix files I downloaded, I can burn as many CDs as I need till I get this to run.

Thanks for your help.