Before we begin, make sure you have Windows Movie Maker installed on your computer. If not, get Windows Movie Maker @Microsoft.

Saving PowerPoint slides as images:

Since Windows Movie Maker support video, audio and pictures as input files, we need to save PowerPoint slides as images first so that we can “import” PowerPoint to Windows Movie Maker.

Step 1: Open your PowerPoint file, and click “File” → “Save As”.

Step 2: In the pop up “Save As” dialog, on the “Save as Type” option, choose one image format. There are 6 image formats you can choose: GIF, JPEG, PNG, TIF, BMP, and EMF. Any one format would be ok. Then click "Save".

Step 3: A prompt will pop up and choose "Every slide" so that you can save all PowerPoint slides as images at one time.

Importing PowerPoint images to Windows Movie Maker:

Now you can import all the PowerPoint images, edit them and create a beautiful video.

Run Windows Movie Maker, and under the “Capture Video” menu, click “Import pictures” to import the generated PowerPoint images on your computer.

Notes:

The disadvantage is that it cannot keep the dynamic effects like animations, transitions, music and video clips in original PowerPoint presentation.

In order to save PowerPoint as video while keep all original PowerPoint elements and effects, you can use third party tool – Moyea PPT to Video Converter. It helps you to convert PPT, PPTX, PPS, PPSX, etc. to popular video formats like WMV, AVI, MP4, FLV, 3GP, VOB, MOV, MKV, etc. with powerful functions and at super-high speed. You are allowed to set video output settings including video/audio codec, quality, video size, bit rate, frame rate, etc. for your specific needs.

Source from: http://www.dvd-ppt-slideshow.com/blog/how-to-save-powerpoint-as-video-using-windows-movie-maker/