Method A:
1.Save your PowerPoint slides as images like jpg or png in Mirosoft PowerPoint
2.Import these pictures into Windows Movie Maker and make a video.
3.Convert the output video to flv with the free video converter

Method B:
Record Powerpoint presentation from screen using screen video capture software

Method C:
Use a PowerPoint to video converter to convert your powerpoint file to flv directly.

Notice: All your PowerPoint original elements like animations,transitions, video clips and sounds will be removed when you save your powerpoint slides as images using Method A.