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You can use Picasa if you already have it installed. It’s very rudimentary, but you can do what you need to do. It works better with some formats that others. A better option would be VLC Player, which is free and open source. In addition to being a great all around video player (I’m not shilling for them, I use it because I like it.), it has some editing features, and editing the video length would be simple.
Here is the link to videoLAN where you can download VLC Player. You didn’t mention what OS you are using, but if you scroll down to the bottom you should be able to find the right VLC version.