>I have been using the Setup Wizard to create an installation program. For performance reasons, I have been installing the app directly on the user's local drive.
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>When distributing revisions, can I have the users re-install the app right over the old one or should I have them uninstall the app from Control Panel first? If we do re-install right over the old copy (and I am sure it has happened), do we end up with a lot of garbage in the Registry (or does SETUP take care of this for us)?

If done interactively, a reinstlal over the top of an existing app is pretty much a no-brainer; if you fire a reinstll silently, from inside a wrapper, there's a command line switch (-r I think) to tell the Setup Wizard that this is a reinstall, otherwise it will stop because of crumbs left behind by the first install.