Unbleto convert to frames

I downloaded a video from youtube ,then i converted to avi format,i used mmreader for reading video i got error as h263 codec must be installed ,i installed it,after that i got following details

General Settings:
  Duration = 5.1200
  Name = new52.mp4
  Path = C:\Documents and Settings\sys\My Documents\Downloads\Video
  Tag = 
  Type = VideoReader
  UserData = []

Video Settings:
  BitsPerPixel = 24
  FrameRate = 25
  Height = 360
  NumberOfFrames = []
  VideoFormat = RGB24
  Width = 640

no of frames is null

kindly say why i get lime this