In this YouTube video, some kind of software is used for generating the voiceover.
I'm using Windows 7. Is there any software available which does this kind of voiceover – but maybe in better quality?
On linux I use festival, where I can easily transform any line into speech (like cat text.txt > festival |tts. On Windows there used to be Microsoft Sam, nowadays called Microsoft Anna. This is a reasonably good text2speech service, unless you are willing to pay for a commercial variant. It should be installed on Windows 7 by default.
Just open the Control Panel, open the Speech Recognition category and go to Text to Speech. Here you can enter any text and get it read to you. You could install any audio capture program to record the sentences and use it as an audio track for your video. If you need more languages or functions you could download Speakonia, but it uses the default Windows speech engine.