first of all, make sure zdesktop.exe is not running (task manager). if it is still running, stop it with Control Panel->Administrative Tools->Services->Yahoo! Zimbra Desktop Service->stop; or just kill it with task manager.
once zdesktop.exe is stopped, backup these folders: derby, store, index, so that you won't lose your email if something goes wrong in the following attempts of restoring zd.
before re-installing zd, do you see .install4j/installation.log under your installation directory? if there is one, please attach it here - that will help us to figure out what went wrong.
now try these steps:
1) download zd installer from
Zimbra offers Open Source email server software and shared calendar for Linux and the Mac and try to reinstall it at the same location.
2) if (1) still fails, follow the instruction here to do a manual uninstall; then re-install again.
Yahoo! Zimbra Desktop FAQ - Zimbra :: Wiki
to restore your email once you have a successful installation, you will have to stop zdesktop.exe again, copy back those 3 folders you backed up; launch zd.