When you try to play a DVD on a computer that has been upgraded to Windows XP, you receive:
Windows Media Player is currently unable to play DVD video. Error ID = 0xC00D116A, Remedy ID = 0x00000000.
This behavior occurs if the 3rd party DVD decoder program that is installed on your computer is NOT compatible with Windows XP.
To resolve this issue, contact your computer manufacturer, or the manufacturer of the DVD decoder program.
NOTE: see 310436 - Cannot Play a DVD in Windows XP.