Windows contains a trap in which quite a few computers seem to get caught sooner or later. The trap was described in a Web article whose link no longer works (and also in another one mentioned below):

PIO mode is enabled by default in the following situations:

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.

In this case, the user cannot turn on DMA for this device. The only option for the user who wants to enable DMA mode is to uninstall and reinstall the device.

Windows XP downgrades the Ultra DMA transfer mode after receiving more than six CRC errors. Whenever possible, the operating system will step down one UDMA mode at a time (from UDMA mode 4 to UDMA mode 3, and so on).

If you're not interested in the details, but just want to fix this problem as quickly as possible:

1. Click here.
2. Despite any warnings click on the [Open] or [Execute] buttons as required to execute the file resetdma.vbs. (If you fear that this web site could be malevolent, you can use the manual method instead, which is described below. Or you could download, save, and inspect the program with an editor like the Windows Notepad. It is a script text file.)
3. If the program found any ATA channel to reset, reboot your computer and test all drives.
4. If the problem is still not solved, set the offending channel to PIO manually, reboot your computer, set the channel back to DMA, and reboot again.

Full article