You are right on the point!!!
XP indeed has reverted my primary DVD drive to PIO (I tried to read a damaged CD before this happened). After 2 hours messing up with my primary IDE channel, now both of my optical drives are "Ultra DMA mode 2". so is this supposed to be the "right mode"?
The reason it took me so long is my XP got stuck with updating drivers for the IDE channel (simply changing the transfer mode in Device Manager does not work, so I have to reinstall the channel). I googled around and found out this thing about "a346bus.sys", so I boot to safe mode without loading it, then changed the transfer mode there. Does this have anything to do with Alcohol 120% ?