I used to own a s710a, but now i have a w600i. Hmm, i used to have videos on mine. Alright, you first need a program, you should get Xilisoft Video converter which is found at http://xilisoft.com/video-converter.html and you just open your movie in there, and select 3gp mode. You can select that and encode with that codec, or you can bumb it up to mpg4, you have to play around with the bitrates and frame refresh rates, i think set the refresh rate to 15, but just play around with the settings, and you'll be able to use mpg4. Then just put the videos on your Mem card Duo in the videos folder.
BenQ converter is also good and is freeware, so is Super Converter. Both and more are available at http://www.FireEgg.com
