I'm not a Windows vidder (I vid on Linux), so my procedure won't be exactly the same as yours, but that's actually good news, because there's a hell of a lot more tutorials out there for doing this stuff on Windows than there is for doing it in Linux.
For what we're talking about, converting your files to editable format, this is one of the available tutorials there: http://elvirasweeney.com/iweb/ElviraSweeney/Making_HD_clips_for_fan_videos.html ... Note that Avidemux has changed its user interface a little bit since the screen captures here, so while you can still do this, buttons may have moved around a bit.
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This website has a lot of great tutorials for vidders, though they're a few years old and may not be 100% state of the art, the principles are the same.
http://elvirasweeney.com/iweb/ElviraSweeney/Welcome.html
For what we're talking about, converting your files to editable format, this is one of the available tutorials there: http://elvirasweeney.com/iweb/ElviraSweeney/Making_HD_clips_for_fan_videos.html ... Note that Avidemux has changed its user interface a little bit since the screen captures here, so while you can still do this, buttons may have moved around a bit.