Please can you tone down the references to "downloading" your .iso's if you don't we can't help you ! We don't condone piracy of any sort here in the forum, well except Pirates of the Caribbean, and if your persist you will have to go elsewhere for help !
My guess would be a format issue as suggested above, if your PC plays them fine but your standalone player doesnt on different media and different .iso's that seems the most likley reason. Where in the world are you ? For instance if you were in the US and these .iso's were in PAL format it appears (from other posts in these forums) that US DVD players and tv's are less happy to play the European TV format...