you *could* use something like the Microsoft Movie Maker or Mac's (superior) iMovie, or even Adobe Premiere Elements or Pro to do this job, but it's alot easier to download AVI Trimmer:
and just do exactly what you want to do on a copy of the clip. quick and easy. unless you plan on doing alot of editing, this is the way to go. edit: make sure you get the 1.2Beta version of that. The 1.0 is more a command line type, give it time values. The 1.1 and beyond are visual editing.