Might be they have a 'regio code'. Hardware in Europe checks if the DVD is for Europe. Hardware in the USA checks if the DVD is for the USA. The world is divided in 5 regios'.
Check the manual for the PC to see if it allows switching regios. There's a risk you can't switch it back, however.
More in http://en.wikipedia.org/wiki/DVD_region_code
Also, of course, there's the PAL/NTSC difference. I don't know how that affects playing on a PC, but it's not unlikely it does.
OS is Win7 with intel dual core. Separate graphics card - the lot. I purchased some rather good compilation sets - BBC and History Channel documentaries - but I want to put them to an external hard drive to play through a media box - but the discs will not even play on my computer with Win7 - or any other computer.
What can be done? When I purchased them, I specifically asked if there was any difference in systems and was assured that there was not. Wrong, of course. I bought them in one of those huge bookshops that you have in the States - can't remember the name though. What I am trying to say is that they are "pucker" sets - not from a street-seller.