For a while now I've had a problem with a windows XP machine having horrible performance. The culprit was that the main hard drive was using PIO instead of DMA. Problem was, when I went to change that in device manager, it wouldn't let me choose DMA - the selection was greyed out! I tried different cables, new bios revision, etc. but nothing worked. I hope if that if anyone else has this problem, they might find this post from a search engine, because I found the fix when Windows XP decides that your drive can't do DMA anymore. Here is the link the answer, read on for more info.
Here is an excerpt that explains what happens:
For repeated DMA errors. Windows XP will turn off DMA mode for a device after encountering certain errors during data transfer operations. If more that six DMA transfer timeouts occur, Windows will turn off DMA and use only PIO mode on that device.