How can I post a PowerPoint Presentation on YouTube?

You will need to somehow convert your slide show into a video.

 There are many software programs that can let you do that...  like 

SnagIT or Camtasia...  you might also try Fraps...

Once it is in a video format, you can post it on YouTube...

If you don't want to spend any extra money, you can recreate your slide show on Microsoft Movie Maker...