Windows Installer MSP file won't install on a restored snapshot due sequencing error, but would install prior to the snapshotting process

Hi

I apologize if this is the wrong sub-forum.  We use VMware 5.1 , with vCenter to test our software application.  We are in in the process of releasing a patch which is delivered via an MSP file.

Scenario 1:

1. Obtain test build of the MSP Windows Installer Patch File
2. Restore VM to "our" clean state snapsshot
3. Install our main application.
4. Install our patch
5. Test...

All ok - Pass

Scenario 2  (designed to speed up testing)

Pre-setup (one time):

1. Restore VM to "our" clean state snapshot
2. Install our main application.
3. Take second snapshot

For each patch:

1. Obtain test build of the MSP Windows Installer Patch File
2. Restore second snapshot
3. Install our patch - FAIL due to Windows Installer sequencing error

The actual error is

MSI (c) (14:44) [13:36:45:194]: APPCOMPAT: looking for appcompat database entry with ProductCode '{4EC8B526-FBAA-4085-9D34-1B6E7A664C1C}'.

MSI (c) (14:44) [13:36:45:194]: APPCOMPAT: no matching ProductCode found in database.

Our main product is installed.  If you search the registry for the ProductCode GUID, It is there.  If I remove the main application, reinstall and then try to run the patch again, it still won't work.  If I jump back to the original snapshot, it works, but won't after the snapshot restore with the main app already installed.

Does this sound like some sort of disk corruption?

Hopefully I was clear about what I am seeing.

Cheers

Tom