Welcome to the forum, Matt
It's probably a region 1 disc then - that's where your problem lies, I think, as most (if not all) machines in the UK are able to play both PAL and NTSC. I'm guessing the UK is where you're from...
What I would suggest, is to backup your DVD (assuming it's copy-protected) with something like DVDFab Platinum, or the AnyDVD/CloneDVD combo - both have fully functional trial versions.
I know a DVDFab-created backup will be region-free, not sure about CloneDVD as I haven't used it in a long time.
But maybe try both and see which suits you best.