Here's some info:
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 the mini-IDE driver for the device does not support stepping down
transfer modes, or if the device is running UDMA mode 0, Windows XP will
step down to PIO mode after encountering six or more CRC errors. In this
case, a system reboot should restore the original DMA mode settings.
All CRC and timeout errors are logged in the system event log. These types
of errors could be caused by improper mounting or improper cabling (for
example, 40-pin instead of 80-pin cable). Or such errors could indicate
imminent hardware failure, for example, in a hard drive or chipset.
http://www.microsoft.com/whdc/device/storage/IDE-DMA.mspx