McAfee SC was in fact uninstalled... but good news, I found out how to remedy the problem
here. Turns out the problem was with WMI (Windows Management Instrumentation), which keeps a database that is apparently not updated properly by Vista. By following the instructions at this link, McAfee no longer appears in SC, and this fix apparently applies to all security software (the user at the link above had the same problem with BitDefender, and I fixed a friend's PC who had Windows OneCare). I had to boot from a PE disk (Bart PE) to delete the indicated folder, however, as I couldn't get Vista to give me "permission" to delete the folder (even in an administrator account and even after entering the permissions dialog and altering them... though I'm still pretty new to the latter and may have overlooked something).
Thanks for the help!
P.S. I followed the instructions manually, didn't use the tool (it's for XP anyway).