Neil, if you are running win7, find the movie file that needs treatment, and RIGHT click it. You will get a drop down menu appear. About half way down is the option to "Open with", which will offer a bunch of choices, one of which should be "Windows Live Movie Maker". Click that and go for a brew.
It should appear with your movie sitting in the timeline/storyboard. Top toolbar, right hand end, is "save", which gives resize options. There is also a box with links to items like YouTube, clicking that gives a set of recommendations, which presumably work. Unless you have really fast PC, anything involving movie rendering can take a lot of time, so its an opportunity to take a boat for a sail.
The program itself is well hidden, living at C:\program files (x86)\Windows Live\Photo Galley , but right clicking the file you want to convert will find it for you. If it really isn't in, it is a free download from MS, just needs a younger person to download and install for you.