Actually, I was wrong, it can be done with free software.
Download Virtual Dub, an Open Source video editing program.
To get this to work, you need to apply at least one filter to your video - since you don't actually want to change anything in the video, you can just apply a "sharpen" filter with a value of 0, so this will be a filter that doesn't do anything.
Then click on that filter in the list of filters and click 'cropping'. Once there you can set the coordinates of the area of the video that you want to use. It includes a preview screen so you can set it properly.
EDIT: oh and here's a link to download Virtual Dub:
http://www.virtualdub.org/