why won't my dvd play?

What region are you buying? Are the correct codec's installed?

Is your DVD player set to the correct region (should be 1 for USA)?

http://en.wikipedia.org/wiki/DVD_region_code

It's possible that if it's a new form of Windows that the Digital Rights Management is preventing these disc's from working. M$ added a more robust DRM Option to Vista and 7 than what XP had and this can cause DVDs to not play.

If you are using XP it doesn't natively support DVDs so you need a add in program that can play them like Nero's Showtime or Cyberlink DVD Player. If you are using the newest versions of these products it's also possible that the Security on the Disc is preventing the Disc from being played in a Computer to attempt to prevent Piracy.

Col