With Stuffit Expander (what I use to expand RARS) I can unpack part of a RAR if I have, say, the first 2 and not the last ones. It isn't finished, but I can watch the first half of the video. I don't know if Winrar does that too.
The problem is, I have part 1 and 3. I cannot watch the last part because Stuffit wonders where rar 2 is.
Yousendit is my service of choice. No download limits. With ady's blessing, I can upload the rars I have on that.