How to cut away parts from .AVI files without any loss in quality

i have a few avi video files,,, some of dem r in widescreen and some not... wat i wanna do is to cut a particular part of dat avi file and delte d rest wthout affectn d quality or d aspect ratio of d video,,, iv tried dis usin NERO bt d output video file size is very very high (coz d main motive of cuttn away a part of d 700 mb video is 2 save space) nd d also also degrades 2 a gr8 extnt along wth d aspct ratio

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please suggst me a free utilty dat cn help me cut away parts 4m n avi file...