As I recall, most CD/DVD players (installed into computers, not stand-alone) can change their regions 5 times before they get hardcoated. At that point, you need to get it reset by the factory. Crazy no?
If you insert the DVD and it still does not play, then there may be some other reason. Normally, you would get a small announcement box telling you about the switching back and forth between PAL and NTSC.s:
This is a link to a site that can help you hack the region code but this is only for stand-alone playerhttp://www.videohelp.com/dvdplayers
I hope this helps.