You can add credits and titles at the start of the video, anywhere in the video and at the end. But as far as I know, you can not use Movie Maker to watermark the entire length of the video. For that you will need some specialty software, such as watermark factory - http://www.watermarkfactory.com/
windows movie maker is not intended for professional use - if you are seriously looking for a program to edit videos for production please at least look at using an editor that can actually save video to professional quality formats. There are several open-source solutions for professional video editing, and there are many tools out there that are free to use. And others that cost money and are worth every penny. Looks like you've got some reading to do my friend!
If you want to put watermark on your video..you can try and check Ulead Video Studio . It look similar to Windows Movie Maker and have more functions.Try to discover it
I mean a professional photographer could use a disposable camera to take what are technically photographs, but there is no way a cheap tool can create a product that is on-par with professional tools. You cannot make or even save a professional file from Windows Movie Maker, and it's not even designed to make professional videos anyway. If you're serious about it and don't want to spend money for some windows software, go get UbuntuStudio, Kdenlive and Cinelerra and come back when you need help with those. They are all free and with those tools you can create professional level files. Don't waste your time and our time with a toy product made for a toy operating system.