Disable the DVD drive autoplay using properties in Windows Explorer.
Right-click the DVD drive, select Properties
Click the AutoPlay tab
Click the dropdown box and select DVD Movie
Click the "Select an action to perform"
Select "Play DVD movie using VideoLAN media player"
Put in a DVD and see if it obeys.
You may have done this, but since there are a few ways to do a similar method, I put this in for redundancy.
You can download the Microsoft Autoplay Repair Wizard HEREor Microsoft Windows XP PowerToys HERE.