OK -- paying close attention this time, here's what I observed.
1. Started by following full uninstall instructions
- instructions:
http://forums.comodo.com/install_setup_configuration_help/internet_still_blocked_after_uninstalling_cis-t42341.0.html;msg307041#msg307041- start with built-in CIS uninstaller, reboot
- run CIS Clean-up BAT batch file, reboot
- look for and delete the listed files and directories (none found)
- look for and delete registtry keys
-- found several LEGACY_* as listed in instructions
-- unable to delete these
-- none of the other listed keys were found
-- tried turning off UAC then rebooting and deleting but I still can't
-- I'm on an admin account ... why would I be blocked from doing whatever I want?
- finished by rebooting
2. Reboot again for good measure
3. Now reinstall CIS 5.10.nnn.531
4. Reboot
- observe no CIS icon in tray
- observe cfp.exe process in Task Manager
- double-clicking CIS desktop icon gets second cfp.exe process but no UI
5. Reboot
- observe same as after last reboot
6. Reboot
- observe same as last reboot
7. Reboot
- same results
8. Uninstall with CIS built-in installer
-- balloon popup "Windows has blocked some start-up programs. Click to view blocked."
-- I clicked, but it didn't show anything
-- clicking the "blocked" icon in the tray didn't show anything either, it just went away.
That's where I am now.
Does anyone have ideas, or am I going to have to reinstall Vista to get this to work / get back to a clean state?
Am I wrong to try and use CIS with Vista 64? Or am I just not doing it right?

As always, thanks for any help.