Text over video doesn't work
You add a video to your presentation. You put some text over it. You play the show and .... where did the text go?
No matter what you do, you can't get text to appear over your video.
In PowerPoint 2007 and previous versions, video always plays "on top" of everything else on the slide, so you can't put text atop a video and expect to see it in slide show view.
PowerPoint 2010 adds the ability to put text and other graphics atop videos, but in order for this to work, the presentation must also be viewed in PowerPoint 2010 (or later, presumably) or in the free PowerPoint 2010 Viewer.
If you have an earlier version of PowerPoint or must ensure that your presentations work correctly in earlier versions, you could use Windows Movie Maker or another video editing program to add text directly to your movie before inserting it into Powerpoint.