with virtual dub just open the program. then open the file you want to work on with it. Use the bar at the bottom of the screen to slide thru the film, click edit (i think) and click set start point. then click to the end of you rsample and click edit > click set end point. Then click file and save segemented avi as...
oh one thing make sure your processing mode is set to direct stream copy otherwise it will re encode the whole sample and take ages instead of a minute or two.
You should be able to figure it out from these instructions, im going from memory here, but its pretty simple anyways
