Several helpful but shy folks sent me private notes with the solution. It is:
-- Click Start / Control Panel / Add or Remove Programs.
-- Check the checkbox that says "Show updates" at the top of the window.
-- Scroll down the list to find Internet Explorer 7 beta -- it will be
nestled amongst other updates you've made. Click Uninstall. You may
see a dialog box warning that other changes made since you installed
IE 7 may be harmed.
-- Restart the computer, and IE 6 is back where it was before IE 7
swallowed it up.
My personal advice: don't install the current beta level of IE 7
unless you absolutely have to, and then only on a sacrificial
computer. I really wish Microsoft hadn't made it impossible to run
multiple versions of IE on one computer.