Can't install McAfee 8.7 or 8.5 anymore

(I posted this over on the McAfee forum, but haven't gotten any
response. I'm thinking maybe it's a Windows problem anyway)

I have a Vista PC that I bought a month or so ago, on which I
immediately installed McAfee Corporate 8.5i off of our University's
website, followed by the 8.5 patch. It did regular scans, and never
found anything.

All was well until the University updated the download to McAfee 8.7i
last week. I downloaded this install package and attempted to install
it over 8.5. No dice: I got past the Administrator account prompt, but
then I get the ff. error msg:

This installation package could not be opened.
Verify that the package exists and that you can
access it, or contact the application vendor to
verify that this is a valid Windows installer
package.

So I then uninstalled my old 8.5, thinking (wrongly) that this was
causing the problem. I still got the same error msg. OK, I thought,
I'll re-install 8.5 from its download file, which I still had laying
around. No--I get the same error msg on the same version that
installed OK before!

Since then, I've tried numerous things, like the steps at
http://support.microsoft.com/kb/555175/en-us. What puzzles me is that
the McAfee installer is an .exe, whereas I thought the Windows
Installer program only got involved with .msi files. And indeed I get
the above msg if I try to launch the installer on the command-line
giving it an .exe arg (msiexec /i foobar.exe, replacing <foobar> with
some real exe file); whereas the installer runs fine with the .msi
files cached in c:\windows\installer\. I tried replacing the .exe
suffix on the McAfee installer with .msi (mcafee87.exe -->
mcafee87.msi), but no difference.

Right now I have the trial version of AVG Internet Security installed,
but that only lasts 30 days. (It hasn't detected any viruses either,
so I doubt that an infection is causing the problem.)

Any other suggestions? Why is Windows using the Windows Installer to
run an .exe? (Or maybe the McAfee installer creates a .msi file on
the fly, and that's what the Windows Installer is supposed to "run"?)

Mike Maxwell