If you have created your lecture in PowerPoint, and want to turn the file into a video with narration, you can do this without any additional software. This feature is available in PowerPoint 2010 and later.
Record a Narration
Under the Slide Show Tab, choose "Record Slideshow. This will launch the Presentation, and will record your narration through whatever is set as the default microphone on your computer (be sure to test that audio is being picked up by external mics). Go through your presentation normally, and it will record the entire session. Be aware that under the same "Record Slideshow" dropdown, you can record over individual slides if you make a mistake.
Save it as a Video
After recording the presentation, click File > Save as > Save as type. You will see an option called "Windows Media Video" or .wmv which will save your presentation as a video. Be sure to save your file as both a PowerPoint file and a windows media file, so you can go back and edit the PowerPoint later.