Another thought occured to me regarding your post, but since we know nothing about your pc's hardware, it's a real stretch for us.
If it's a factory pressed dvd, and you can play other DVD's on both your pc and set top box, then region code is not likely. Have you tested other DVD's you can play on set top in your pc?
Is the DVD in question truly a factory pressed CD or is it a burn? Some companies do release "legal releases on burned media".
If it's a burn, is media compatible with you pc's drive?
Can you simply not play the DVD as a movie..but your system can see the disk in the drive? Or, does your system not recognize a disc is in the drive at all?
What programs if any are installed on your pc to watch a DVD, and if so...what apps do you have running in the backround, they may be causing a confict.
Sorry for such a long post, but your question leaves us with alot of questions before we can be much help...