When I tried to play a DVD directly on my computer the sound and video was very jerky. I figured I'd fouled something up when I was turning off Autoplay on my CD-ROM the week before last to get around the "Macromedia Splash Screen" BS that Motorola put on the CD-ROM with their cable modem to make it impossible to use the CD-ROM.
I went down my usual false trails of thinking it was an issue with the VideoLAN VLC media player I use. It wasn't. Checked that by copying the DVD to my hard drive. VLC played it fine from there.
Did some more searching and found the answer thanks to various kind people who'd posted stuff about it.
It turns out that on Windows XP machines there is a bug. If the machine is having trouble getting data from the CD-ROM it drops down to an older and simpler data transfer protocol, "PIO". And then it gets stuck and won't switch back to the faster "DMA" protocol.
There are various ways to fix this, and you may have to try several to get one that works for you.
Anyway, here are the web pages I found helpful. My sincere thanks to the people who wrote them.
(What happened to the button for creating URLs here? What have I messed up now?)
(Edit: Those buttons are only in the "Rich Text" mode, not the "HTML" mode. But they weren't showing up for me when I was switching before. Maybe the net was laggy and I was too impatient.)
Disclaimer: I'm just a half-assed dilettante messing with this stuff. Don't mistake me for someone who actually knows what he is talking about. Use at you own risk. May contain nuts.
(Hey! The Firefox spell check right click menu works right in here now. Thanks to whoever it was that fixed it.)