Machete Lite is the easiest to use free app I know of that does it without re-encoding and therefore has no loss in video quality. I think Avidemux and Virtual Dub also don't re-encode, but they are not as easy to use and may not work on WMV files?
Very easy to use. You load the file, select the beginning and end points of the cut and then hit the delete key and resave. Very fast.