So - how can I play the video files back on my Sony Ericson k750i?
Try recoding them with something like Super (free) from
http://www.erightsoft.com
Its a pain in the ass to find the download link usually, with multiple
pages of spiel telling you what it can do (and it does _the lot_) but
hopefully this will help you get there quicker...
http://www.erightsoft.net/S6Kg1.html
Scroll right down to the bottom for the download links...