maybe not the easiest buy virtualdub is fairly easy for avi:
1. open file and use the slider at the bottom of the screen. to cut out the part you don't want, slide to where this unwanted portion starts and press the the "mark in" button on the bottom. then go to the end of the unwanted portion with the slider and press "mark out"
2. the unwanted portion should now be highlighted. press the del key or edit->delete
3. after finishing that from the menu select:
video-> direct stream copy (to ensure video is not reencoded)
file-> save as avi
for mpg/wmv look at the tools here:
http://www.videohelp.com/tools?s=8#8
there are freeware and shareware to do this, most of which i haven't tried. for mpeg, videoredo is easy but not only a trial version is available. otherwise, Cuttermaran may be a good choice as it's free and is updated frequently.
for wmv, choose for yourself from those tools as I'Ve never edited a wmv myself