|You probably need to install the motherboard chip set drivers in order to get the drive to run in a DMA mode.|
If that isn't the issue then one one of your drives may be failing.
WinXP will automatically step down the transfer speed if the drive or controller can't keep up. The usual cause of that is a failing drive but could be a loose or broken data cable.
Most likely the chip set drivers though.
I can't steer you to the drivers because you didn't post your system specs as instructed when starting a thread here.