Thanks for the advice, guys and/or girls.
To do what I needed to do (take youtube video, get rid of the soundtrack and dub in my own), I've worked this way, if anyone else needs to do it:
Download vids with Youtube downloader.
Load them into Windows Movie Maker, turn down the audio and just overlay my own .wavs from Audacity.
I also needed to convert audio recorded by Windows Sound Recorder to .wavs so Audacity could work with them, and used Virtual Dub to do that.
Prism video file converter has also been handy.
All this stuff is freeware and I've had no problems with any of it so far.
Cheers