Grr. Windows XP will downgrade an ATAPI DVD/CD-ROM from Ultra DMA Mode 2 (which is good) to PIO mode (which eats tons of CPU and generally sucks) upon receiving “repeated DMA errors”. Unfortunately, the DMA errors could be due to inserting a crappy CD. End result: the user (me) can’t use his DVD drive to play back movies until he “reinstalls” the Secondary IDE driver. Once I did that, everything is back to normal. Here are the details from the horse’s mouth.