All I can suggest is you put a complete list of your hardware, I have 2 DVD readers Toshiba and JLMS, sometimes the Tosh won't read a disk so it goes in the JLMS.
but it also sounds like something might still be going on in the background (virus checker or defrag etc.) that is causing this.
I would test it on another system (if you can) as you only need a DVD reader to use CloneDVD / AnyDVD, then if it works on that you might consider wiping your system (I know I hate that too) and re-installing XP then trying CloneDVD out at the very start,
after that keep a written log of anything you install and test CloneDVD out after each new installation, this is possibly the only way you will ever find out what is going on, also do a Virus check to make sure you haven't developed a case of worms.