I'm not really sure whats causing your problem but you can try these things to see if you can create a playable DVD on a set top box.
Try switching your burn engine to ImgBurn in your Settings/DVD/Write options. It's a free ISO Image burning software available at http://www.imgburn.com/
I use Vista home premium 64 bit and was haveing trouble playing disks made on my set top box and doing this fixed it right up.
You can also try the built in VSO Engine but I think that ImgBurn does a better job.