i too once thought of how i could do this
what i ended up thinking was the best way was to use a screen capture tool (ie camtasia studio).
but since i hate those programs and they're crap...
what i ended up doing was record to a vhs tape my tv output while running the ppt presentation.
granted, this didnt give me the greatest quality (possibly cuz i got an old 80s vcr) but still...its better than camtasia imho.
hope that helped..