I understand the customer wants this but this seems IMHO totally wrong.
Using ZCS allows to have all messages in a single place (server) with SIS (and HSM & deduplicated backups if you're using NE).
Using Outlook+ZCO or zDesktop will create a local (in each user profile) copy of all the data.
While this is nice for roadwarriors and laptop, it's an awfull waste of space and ressources on a TSE farm, isn't it?
Once again, just my own point of view on this...
To answer more on your question:
. zdesktop 1.0.x installs itself in the user profile (default)
. zdesktop 2.0.x installs its binaries in the "program files" and the data in the user profile
You can trick zDesktop 1.0.x to install "elsewhere" while you can't yet with 2.0.x. See :
https://bugzilla.zimbra.com/show_bug.cgi?id=43474