I have 1 HDD master IDE 1, 1 DVD ROM as slave, 1 HDD as master on IDE 2 with CDRW as slave, 1 HDD on IDE 3 (highpoint controller). Make sure jumpers are all correct for master and slave.
Make sure software is looking at the correct drive to run a dvd from.
Uninstall all extra dvd players (software) and use one in XP.
Is the region code locked? and are you trying to play a different region DVD?
Maybe a case of backing up important files and formarmating and reinstalling xp.