Check with your motherboard manufacturer that there are no known conflicts with the hardware that you have.
I think XP has a lot to do with this as well - I tend to have similar problems mainly because of driver conflicts between XP and the manufacturer - it is best to use the manufacturers for obvious reasons - dont let XP try and take over.
It might be worth checking your virtual memory and page file settings (including cache) - even though you are copying on-the-fly, files do still become cached.
Have you tried copying an audio or data CD - if so do you still have the problem ?
Try some different software like DVD shrink or something?
Maz