Question

How can I get the videos of Prof. Walter lewin downloaded on my PC??

Answer 1

Go to a video, say:
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-27/

then click on [download] word under the video and right click on the the link and select "Save as"

Answer 2

Use Youtube downloader !