The fact remains, I had installed Avast 7. I installed the rollup (I think, I installed a hotfix that was listed as being required for W2K, anyhow), and reinstalled Avast 7. Reboot. Still no good. Uninstalled and tried Avast 6, no luck. Reinstalled 7, still no good. Installed the rollup (again?) , and as soon as the update finished up, poink! there's a popup welcoming me to Avast. No other software changes done at all. And now it works normally. Perhaps there's something in the rollup that doesn't always take? Maybe you've got to do the Rollup after installing Avast 7? I don't know, sometimes with Win it seems like you have to do things multiple times before they take.
I dunno about Version 6. I don't know if I've ever tried installing that on a W2K computer (before today, anyhow).
This all was on an absolutely fresh reformat and install of a slipstreamed W2KPro SP4 (the computer is an old junker being prepped for somebody who just needs email). The only other software installed (aside from a couple of tries with other AV progs, subsequently uninstalled) was Firefox. There wasn't any point in installing anything more until I could get a working AV.