Error 1335 in MSI

I'm at a loss as to where to ask about this, so I'll try here:

We created an MSI file to install a VB.Net application via the web. We put
the MSI file on our website, and created a simple webpage with a link that
points to the MSI file. When the use clicks on the link, IE asks if the user
wants to save it or open it. We advise the user to click open, and IE
downloads the MSI file to the temporary internet files folder and then
launches the installer and installs the app.

Most of the time this works great. But occassionally a user receives error
1335 when the installer is attempting to do it's job. This error reads:

The cabinet file '_93FDCFE523EF6D301BLAHBLAHBLAH' required for this
installation is corrupt and cannot be used. This could indicate a network
error, an error reading from the CD-ROM, or a problem with this package.

The CAB file has some long GUID looking name. I have no idea where this CAB
file came from or what it's all about. The application itself is very simple
:
a single EXE, a few dozen JPGs, and 2 DLLs: AxInterop.mediaplayer.DLL and
Interop.mediaplayer.dll (the app uses media player to play video files from 
a
server).

I've been trying to analyze this and found that when the MSI file is
downloaded through IE, it gets changed. I did a binary comparison of the
original with the file after IE downloaded and saved it locally, and found
that several dozen bytes changed. Usually the change is that the high-order
byte gets toggled, for some reason. I can provide the output from file
compare if that would help.

The rub is, the changes cause it to fail. So far, inside our LAN/Domain,
this failure occurs on every computer, regardless of OS (W2K or XP, SP2 or
no). It has failed once on a computer outside of the local domain, which was
XP (no SP2).

In another forum, someone suggested making sure that the IIS MIME setting
for the MSI file did not try to cast it as a text file. The MIME setting is
"application/octet-stream".

What can I do to eliminate this error on ALL computers in ALL environments?