I know this may sound silly but.........
Go into your BIOS setup and go to the page with the drives listed. Tell it to scan for the drives hooked up to the computer. Make sure that when they come up, they still read like they are supposed to. In other words, make sure the Hard drive still shows as the master, and the CD ROM still shows as the slave. I know, you would remember if you did something like move the jumpers, but I've had a BIOS actually lose the information and get it reversed. I had to physically unplug the drives, scan the motherboard in the BIOS, then hook them back up and scan it again. Dont ask me why.
Only other thing I can think of is that you either have a bad or corrupted driver for the CD ROM in your XP setup, or you have an IRQ issue. Sorry I cant be more help, but as anyone in tech support can tell you its hard to diagnose problems like this long distance. Esp. since I'm not familiar with your specific hardware interactions.