Well, I use this program I got free through mail, Powerdirector Pro (30 day trial here (http://www.gocyberlink.com/english/download/download.jsp)), but you can also use Windows Movie Maker, which might not give you the best quality (unless you have version 2, which lets you decide on the size to a much greater degree) but does the job of clipping.

How to clip is different from program to program, but most of the time it's pretty easy to catch how it works, they're deliberately made very user friendly.