I was pulling my hair out on this one for several days

, so I'll post my solution, hoping someone will run across it with search.
I knew AviSynth worked on my Windows XP box, as I had tested the Version.avs and other samples.
It was an older version though, so I unregistered avisynth.dll, deleted the directory and reinstalled with latest 2.07 installation file.
But I kept my Plugins directory, with the various plugins I had already downloaded. And I set the registry key to point to the directory.
And then I couldn't open any .avs files in anything, not even WMP. I tried three solutions involving a TmpGenc patch, AviSynth patch, and changed to the VFAPI environment settings, nothing worked.
Turns out I also kept a few of the sample .avs files that had come with a recent plug-in download -- they were in my plugins directory. Bad news!
The lesson?
Don't keep any .avs scripts in your plugins directory, or AviSynth scripts won't load in any app!
(Thanks to
Zep's post on Doom9, I figured this out from that...)
PS: One error I got in Windows Media Player (WMP) when opening .avs files was "your computer is running low on memory"; even though there was over 300 MB free at the time. That could be a hint to check the plugin dir...