I think this is part of the resolution though I'm not entirely sure why only some client sites seem to ever have install errors (I suspect it has to do with a combination of their network, rights, anti-virus).
Possible Resolution:
InstallShield cannot complete its process because the registry or the InstallShield files are incorrect, damaged, or in use. This may result from multiple installations of the same version, multiple installation attempts, or network file locking issues.
Complete one or more of the steps listed below to resolve this issue. Start at the top of the list, and reinitiate the installation or upgrade process after completing each step. Continue with the list if the error recurs.
Run Pvxcom.exe.
Sage MAS 90 or 200 version 4.20 and later:
Pvxcom.exe is located in the Common Components folder: C:\Program Files\Common Files\Sage\Common Components\
Sage MAS 90 or 200 version 4.10 and earlier:
Pvxcom.exe is located in the ..\Mas90\Home\ folder
Reinstall the program. Select the Install a new instance of this application option.
Reinstall using the Windows Add or Remove Programs option.
Rename the Runtime folder located at:
C:\Program Files\Common Files\InstallShield\Professional\Runtime\
Rename the Uninstall Information folder located at:
x:\..\MAS90\Uninstall Information\
Note: Registration keys must be reentered.
Remove multiple installations of the same version of Sage MAS 90 or 200 if they exist. Only the primary installation should remain on the computer.
Note: Use extreme caution when editing the registry. If the registry is edited incorrectly, Windows may not function properly or may not run at all. Sage Software Customer Support is not responsible for assisting with editing the registry. Always back up the registry before accessing it. If necessary, seek the assistance of a qualified computer technician.
In some instances, it is necessary to remove entries in the following registry subkey: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstal\
Before proceeding with this final step, e-mail the setup.ilg file to Customer Support. The file is located in the ..\Mas90\Uninstall Information\ folder. Customer Support will assist in determining that all other methods are exhausted.