I downloaded a "one piece" anime, episode 348 on bitlord, mp4 format f!
Question: what will i do to play the video?
Answers: what will i do to play the video?
The MP4 file you downloaded was probably made with a codec that Winamp doesnt support. When it comes to downloading video files and watching anime, I use VLC Media Player. You can get it here: http://www.videolan.org/vlc/
You can pretty much throw any kind of video file at it and it'll play no problem. On top of that it's a small program.
As AngelusD commented the codex is most likely the reason. VLC is a good program with lots of options but it has issues with the subs on some files (it will substitutes a generic font for any it doesn't recognize). I use several media programs, including VLC, but prefer using Zoom player which doesn't have the font problem. Either program will resolve your issue.