If you only want to cut down videos you have already downloaded, all you need is this simpel tool:
http://www.boilsoft.com/videosplitter/
It will chop what you have anywhere you tell it. VERY easy to use. There is no need to load a video editing program, edit, then render out because not only is that a waste of time, but you will lose quality when you re-render a rendered (compressed) video.
On the flip side, if you download a lot of clips that you want to join into one movie...
http://www.boilsoft.com/avi-mpeg-rm-joiner.html
Same company, just as easy.
Hands down these are the best tools for doing what you want.
Trust me on this one.