While searching i found a site from where you can download all Lectures from youtube or any other video from youtube. so i decided to share with you.
For the purpose of download Lecture do the following steps:
1. First of all reach the particular lecture which you want to download, for example you want to download CS301 Lecture no.01 so for this purpose first of all open this site http://www.youtube.com/ after opening the site in the search box type the words "CS301_Lecture01" you will get the CS301 lecture and play it
2. After play the lecture which you want to download, copy that lecture URL from the address bar. For Example. this is the URL of the CS301 Lecture no.01 "http://www.youtube.com/watch?v=3oWLX5ATXhM"
3. After copy the URL from youtube, open this site http://allvdo.noads.biz/vdo/ and paste that URL into the Green box which you will see on this site contains the wording "Enter URL of video Page".
4. After paste the URL into the box click on the button "Download"
5. In the next step, site will give you download detail which contains:
Links found on http://www.youtube.com/watch?v=3oWLX5ATXhM
Download video.flv - High Quality (320p)
Right-click download link and choose 'Save Link As...' to download and rename file to video.flv"
6. And in the last step, click on the download link which will be given in the download details. I also highlight that point in 5th step.
7. Your downloaded will be started.