This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
The culprit usually is a virus. Do get a recent virus scanner.
If that turns out negative, it may also be DOS (real-mode) driver that loads in the CONFIG.SYS or AUTOEXEC.BAT, or an old version of EZDrive/Disk Manager loading from the MBR.