Well it isn't happening to everyone, that's the problem, why only certain setups? Technical Support is probably better equipped than we are to troubleshoot this sort of thing. I have McAfee's latest 12.1 sitting in XP Pro SP3, Vista Ult SP2, Win 7 Ult SP1 and Win 8 (last 3 all x64) quite happily doing its thing and no problems at all.
I have to ask, have you been running any Registry cleaners lately? This sort of thing is often caused by that. See if there's a restore function if so.
I had the same problem on two of my XP machines. I saw it first about 3 weeks ago.
It seemed the update from 11.6.443/15.6.238 did not go well to the newest 12.1 and 16.1.
Last weekend however a new update finally fixed both machines.
(We now have the Thunderbird-attachment-scan-problem, but at least the firewall is up again.)
I guess McAfee fixed their update process...