The Single File Scanner is associated with "all files", but not with a particular file type. If a particular file type has an application associated with it, Windows should choose that associated application.
.msi for example should be associated with the Windows Installer, through the following registry entries:[HKEY_CLASSES_ROOT\.msi]
@=Msi.Package
[HKEY_CLASSES_ROOT\Msi.Package]
@=Windows Installer Package
[HKEY_CLASSES_ROOT\Msi.Package\shell]
@=Open,Repair,Uninstall
[HKEY_CLASSES_ROOT\Msi.Package\shell\Open]
@=&Install
[HKEY_CLASSES_ROOT\Msi.Package\shell\Open\command]
@="%SystemRoot%\System32\msiexec.exe" /i "%1" %*
These settings (rough draft, not exact copy as for quotes etc.) make sure that .msi files are opened with Windows installer, and are system default on Vista seen as above, and the same on XP 64 (haven't checked older ones right now).
It would be interesting to know where they differ on machines where SDFiles.exe opens instead!